tactical, sharp openings are best for beginners and lower intermediates (like me), because they teach tactics
Best first opening repertoire for beginners to improve

a) No repertoire until you are 1900 FIDE or double that in tactics, just opening principles until then.
-and-
b) 1.e4 as White. Classical repertoire as Black, playing 1...e5 against 1.e4 and 1...d5 against 1.d4, with many vouching for the QGD as Black.
Beyond move 1 whether White or Black in the open games (1.e4 e5) or move 2 (QGD) in queen’s pawn openings, advice gets murkier.
For instance, is the descriptor next to each of the following openings below a good reason to play or a good reason not to play the particular opening for the learner you are referring to?
1) Ruy Lopez: strategical/theoretical complexity
2) Scotch Gambit: concrete
3) Four Knights as White / Petroff as Black: simple
The answers I have heard to the last several questions in your opening post are:
Mainlines, classical, open, tactical, and yes.
The real reason for my post, however, is ... following.

"No repertoire until you are 1900 FIDE or double that in tactics, just opening principles until then"
What does a "repertoire" mean?! If I play the English as white and the French and the Dutch as black, it's already a repertoire, right? I'm far from being a 1900 FIDE, I usually don't learn more than 5 moves (I learn as you suggested the principles), but I do need to know what to answer to a first move!
Your comment isn't clear.

If you hold a hot take like “adopting a system (KIA, Colle, etc.) is a bad idea because it would sacrifice the long term improvement for the short term“ to be axiomatic,
then why bother asking us for advice in the first place?

If you hold a hot take like “adopting a system (KIA, Colle, etc.) is a bad idea because it would sacrifice the long term improvement for the short term“ to be axiomatic,
then why bother asking us for advice in the first place?
I read multiple times that adopting a system sacrifices the long term for the short term improvement because it makes beginners play robotically on auto-pilot without taking into account what their opponent does so it doesn't teach them the general opening principles. So I thought it was an uncontroversial fact, but perhaps I was wrong. Please do share your line of reasoning if you disagree.

If you hold a hot take like “adopting a system (KIA, Colle, etc.) is a bad idea because it would sacrifice the long term improvement for the short term“ to be axiomatic,
then why bother asking us for advice in the first place?
I read multiple times that adopting a system sacrifices the long term for the short term improvement because it makes beginners play robotically on auto-pilot without taking into account what their opponent does so it doesn't teach them the general opening principles. So I thought it was an uncontroversial fact, but perhaps I was wrong. Please do share your line of reasoning if you disagree.
I agree with the sentiment expressed by Pfren in this thread, that a beginner’s chess studies shouldn’t focus on the opening, easily the least relevant part of the game to their results.

Much like the OP’s concern about the “best first opening repertoire”, I have struggled to decide on move 2 of the Open Games as Black whether to play 2...Nf6 or 2...Nc6, and why. Although both are sound and played at the amateur and GM levels in chess, I would imagine that playing and analyzing one’s own games in these “openings” would lead to a different learning experience after 7-10 years.

Firstly, as I already said in the first post:
"The players under consideration here, the so-called "beginners", are players who are weak but not so weak that learning opening theory is useless for them, players who have barely attained the level at which starting to learn some opening theory becomes useful, so roughly around 1400 Elo."
Perhaps you understand the word "beginners" differently than I do. Perhaps a more appropriate word would have been "amateurs" or "novices" or "club players" or "intermediate players" I don't know. By "beginners" I don't mean people who have just learned how the pieces move.
Secondly, nowhere did I suggest that beginners should spend a large amount of time learning openings, and I know that this would be ill-advised. I am not asking for a 10 moves deep repertoire, but merely a 3 moves deep repertoire. It won't take much time for beginners to learn a 3 moves deep repertoire, merely 5 minutes. And after the first 3 moves the beginners will already be out of their book and will need to start thinking on their own. In the eyes of strong players a 3 moves deep repertoire might look too small to be called a "repertoire", but for beginners this can definitely be called a repertoire. An example answer could be "with White 1.d4 2.c4 3.Nc3, with Black the KID 1...Nf6 2...g6 3...Bg7 4...d6 and the French 1.e4 e6 2.d4 d5", see, short and simple.
The idea was not to give beginners a repertoire in order for them to get an advantage out of the opening, but to give them a repertoire in order to lead them to some types of positions (tactical, positional, open, closed, classical, hypermodern, sharp, slow, I don't know which type would be best) which will be particularly instructive to them and will make them improve faster. To give an example: I've heard some people say that beginners need to play tactical openings because tactics is the most important thing that beginners need to learn, and these people would typically suggest the Sicilian or the Double King's Pawn and disapprove of the Caro-Kann and the London.

Beginners love openings, so telling them not to learn openings will just put them off chess. That's not what anyone wants.
What you really want are openings that they can keep playing forever, but which lead to positions where normal developing moves are the order of the day. If they want to change openings later then they can, but you should give them stuff that will serve them for a lifetime if they want it to. Obviously, open positions are also the priority.
So, you teach them:
White:
- Scotch Four Knights - just the first few moves. 1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. d4 exd4 5. Nxd4 and then lay out the plans for both sides. Often usable against he Petroff too.
- Open Sicilian - again, just the first few moves 1. e4 e5 2. Nf3 intending 3. d4 and 4. Nxd4 and 5. Nc3 and again lay out the plans for both sides. Tactically, the benefits of playing the open Sicilian massively outweigh the costs, in terms of how much they will learn.
- Exchange French with immediate c4: 1. e4 e6 2. d4 d5 3. exd5 exd5 4. c4. Explain the basics of IQPs.
- Exchange Caro Kann - Panov-Botvinnik: 1. e4 c6 2. d4 d5 3. exd5 cxd5 4. c4. Again, explain the plans.
Black:
- Petroff 1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. d4 d5 and 1. e4 e5, 2. Nf3 Nf6, 3. d4 exd4. 4. e5 Ne4 5. Qxd4 d5 6. exd6 Nxd6.
- Tarrasch 1. d4 d5 2. c4 e6 3. Nc3 c5 4. cxd5 exd5 5. Nf3 Nc6. Again, explain the basics of IQPs.
- Attempted Tarrasch against 1. c4: So, 1. c4 e6 intending 2. d4 d5 3. Nc3 c5, and if 2. Nc3 d5 intending 3. d4 c5.
This is the optimal repertoire for beginners because it's future-proof but also exposes the player to a range of different open positions. The amount they need to know to get playable positions is minimal.
You wouldn't go into more depth than the above list of moves - instead, you'd immediately start showing them Grandmaster Games with these openings, and analysing them together. And focus on the tactics.

"Beginners love openings, so telling them not to learn openings will just put them off chess. That's not what anyone wants. " Well said.
Why knowing how to manage with an IQP is essential for a beginner? I personally simply avoid openings and variations leading to such pawn structures, and I think attacking without having to care about structural weaknesses is easier (and it is possible to have an outpost for a knight without an IQP).

What Skynet wrote may come closest to to answer, i d give but struggle to formulate.
What "beginners" or what I d call "weaker players" like myself consider an opening repertoire is i think different from what strong players or masterlevel players call a repertoire.
For us it s usually just a few moves to open the game and get smth we are comfortable with, even not necessarely knowing every intricacy or plan of a position.
And let s be honest, even for the stronger players, it doesnt really matter if a certain opening qualifies as equal or slightly better for one side.
Having fun and beeing comfortable is way more important for me.

If you define "beginner" as 1400 then they already have a repertoire. I mean, surely someone could reach that skill without much of one, but in the age of the internet, chess videos and games you can watch all over the place, etc, if you teach an opening repertoire to a 1400 player, it will certainly not be the "first" opening repertoire they've ever had, and therefore I'm confused about the whole premise of this thread

As far as openings go, I have always believed that the passion one has for an opening matters more than anything else. If you're just dying to play 1.b4, I say go for it! The excitement you have about a variation can sometimes make up for its "sketchiness." I played an unsound variation of the Dragon as a beginner, but since I was so passionate about the opening, I found a way to make it work. Once you cross 2000, you do need to put a considerable amount of work into your openings and make sure you know the plans, but if you do this you'll be just fine!
That being said, if anyone is interested in learning more about the Caro-Kann or Dutch Defense with Black or 1.d4 lines with White, I just uploaded a video of me playing 10+0 games with live commentary and analysis (Here’s the link: https://youtu.be/O1kOYk3FnPw). Hopefully this helps improve your game! Every subscription means a lot to me, so if you subscribe, I’ll really appreciate it.
YouTube: https://www.youtube.com/channel/UCpRYz_ElTJC-FUq4unehOfg/
Twitch: https://www.twitch.tv/giraffe_chess/
I keep coming back to "pick a player and learn openings through their games" and not just for beginners.

Beginners love openings, so telling them not to learn openings will just put them off chess. That's not what anyone wants.
What you really want are openings that they can keep playing forever, but which lead to positions where normal developing moves are the order of the day. If they want to change openings later then they can, but you should give them stuff that will serve them for a lifetime if they want it to. Obviously, open positions are also the priority.
So, you teach them:
White:
- Scotch Four Knights - just the first few moves. 1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. d4 exd4 5. Nxd4 and then lay out the plans for both sides. Often usable against he Petroff too.
- Open Sicilian - again, just the first few moves 1. e4 e5 2. Nf3 intending 3. d4 and 4. Nxd4 and 5. Nc3 and again lay out the plans for both sides. Tactically, the benefits of playing the open Sicilian massively outweigh the costs, in terms of how much they will learn.
- Exchange French with immediate c4: 1. e4 e6 2. d4 d5 3. exd5 exd5 4. c4. Explain the basics of IQPs.
- Exchange Caro Kann - Panov-Botvinnik: 1. e4 c6 2. d4 d5 3. exd5 cxd5 4. c4. Again, explain the plans.
Black:
- Petroff 1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. d4 d5 and 1. e4 e5, 2. Nf3 Nf6, 3. d4 exd4. 4. e5 Ne4 5. Qxd4 d5 6. exd6 Nxd6.
- Tarrasch 1. d4 d5 2. c4 e6 3. Nc3 c5 4. cxd5 exd5 5. Nf3 Nc6. Again, explain the basics of IQPs.
- Attempted Tarrasch against 1. c4: So, 1. c4 e6 intending 2. d4 d5 3. Nc3 c5, and if 2. Nc3 d5 intending 3. d4 c5.
This is the optimal repertoire for beginners because it's future-proof but also exposes the player to a range of different open positions. The amount they need to know to get playable positions is minimal.
You wouldn't go into more depth than the above list of moves - instead, you'd immediately start showing them Grandmaster Games with these openings, and analysing them together. And focus on the tactics.
That's the kind of answer I was looking for. I agree that the openings you recommend are very open. I agree with the idea of giving beginners "openings that they can keep playing forever [...] that will serve them for a lifetime if they want it to", but IMO most of the openings you recommend do not fit this criteria. The Open Sicilian is the only one that is optimal. The Petrov and the Caro-Kann Panov-Botvinnik Attack are borderline. The Tarrasch, the Scotch Four Knights and the French Exchange Monte Carlo (4.c4) (a QGA a tempo down...) are objectively bad and are currently extremely rare among GMs.
Instead, the openings that would best fit the criteria of "they can keep playing forever", i.e. those that are the objectively best openings and the currently most popular among GMs, would actually be:
For White:
- the Ruy Lopez, 27 times more popular than the Scotch Four Knights
- the French 3.Nc3, 120 times more popular than the Exchange Monte Carlo (4.c4)
- the Caro-Kann Advance Short, 8 times more popular than the Panov-Botvinnik Attack
For Black:
- 1.e4 e5 2.Nf3 Nc6, 9 times more popular than the Petrov
- as for the Tarrasch, there are only 30 Tarrasch games (including both 1.d4 d5 2.c4 e6 3.Nc3 c5 and 3.Nf3 c5), compared to 1000+ Semi-Slav, 700+ QGD ...Be7, 700 QGD Ragozin, 2000 Nimzo, 1200 QID, 1000+ Gruenfeld, 1000+ KID, etc.
The previous Chessbase statistics are among GMs between the year 2014 and 2020.
It sounds like you already know what you want. Why not just search for openings that are most in fashion among GMs and teach those? They wont be bad and you'll have a lot of recent material. As a wise man once said, "Teaching is like sex- ethusiasm is more important than technique!" If you believe in the choices your students will pick up on it and study harder too.

For a lower level player who never ever wants to switch their openings ever, and plans on improving as rapidly as possible, is it better that s/he chooses early moves that could potentially steer his or her games in the direction of the opening choices of the current crop of top-level GM’s or are there better early move choices that might not be as objectively good, but are clearly better and more efficient for helping develop such a player’s fundamental skills in chess?
What are the best openings that should be taught to beginners in order for them to improve fast and become strong?
Here "best" doesn't mean best to maximize their winning probability on the immediate short-term, but best for them to improve and eventually become strong players. Only the long-term improvement matters. The short-term improvement is irrelevant. I have heard that adopting a system (London, Colle, KIA, etc.) would be good on the short-term but bad on the long-term, because it would limit the range of positions that they would get exposed to.
My goal is not for them to get an advantage out of the opening, but to give them openings that will often lead them to some types of positions which will be particularly instructive to them and will make them improve faster.
The players under consideration here, the so-called "beginners", are players who are below 1100 Elo, players who do not yet know what openings to play, players who do not know anything about openings.
Should each individual beginner play openings that fit his own particular style or preferences? Or is there some particular type of openings that is best to play for all beginners regardless of their individual style and preferences?
What kind of openings should we teach them?
- mainlines or sidelines?
- classical or hypermodern?
- open or closed?
- tactical or positional?
- sharp or quiet?
Most people say: mainlines, classical, open, tactical, sharp.
Most people say: 1.e4 e5 (the Open Game, aka the Double King's Pawn) with both colors. Okay, but what should we teach them to play on move 2 and move 3? And what should we teach them to play as Black against 1.d4, 1.Nf3 and 1.c4?