Good Openings? for Beginners

I recommend chess opening principles rather than memorizing specific openings:

https://www.chess.com/blog/KeSetoKaiba/opening-principles-again 

If you do go with specific opening variations, I recommend the common mainlines (1. e4 e5 and 1. d4 d5 are very popular and worth learning some basic opening ideas in those).

KeSetoKaiba gave some good advice. Personally, openings is what originally got me hooked on chess. When I first started playing I learned so many opening traps that I never got a chance to use. 

So don’t be like me. If I could restart my chess journey, I would follow KeSetoKaiba’s advice from the start. But, if you are set on learning a particular opening, I would recommend something like the italian game. It has very basic ideas and is easy enough that you will probably face the main line for the first few moves.

@1

Probably Ruy Lopez 1 e4 e5 2 Nf3 Nc6 3 Bb5, for white as well as for black.

the Italian game allows you to easily gain the center, develop your pieces, and castle.  you can also play it aggressively with moves like ng5 after nf6 and maybe even go for the fried liver attack.

I'd recommend 1. e4 e5 and 1. d4 Nf6, personally. Queen's Gambit as Black seems like somewhat of a lost cause, compared to Gruenfeld, KID, OID, et cetera.
On a similar note, I'd recommend hypermodern opening principles as Black, and traditional opening principles as White. The tempo White gets (usually to push e4 d4 or c4) already gives White a space advantage, so I'd recommend letting White take a large center and then counter-attacking it with fianchettos and stronger defensive pawn structures.

1. d6/e6/g6/Nf6/c6/c5 as Black against everything, never play 1. d4 d5, never play 1. e4 e5.
1. e4 d6 Pirc.
1. d4 d6 transpositions to OID/QPO/Indian Games or Pirc.
1. e4 e6 French
1. d4 e6 transpositions to QGD/QPO/Indian Games or French
1. e4/d4 g6 Modern
1. e4 Nf6 Alekhine
1. d4 Nf6 Indian
1. e4 c6 Caro-Kann
1. d4 c6 transpositions to Slav/QPO/Indian Games or French
1. e4 c5 Sicilian
1. d4 c5 Benoni

1.d4 d5 is better than Nf6 at low levels because you shouldn't play hypermodern/Indian openings at low level.

@ShuckleSquad13 is correct that I don't recommend playing "hypermodern" openings because they violate the chess opening principle of fighting for the center (short-term anyway). Hypermodern openings are actually in alignment with chess opening principles and the fight for the center IS present in these openings, it is just that the fight is much more long-term and advanced complexity; hypermodern openings like the Grunfeld Defense or the Alekhine's Defense, I probably wouldn't recommend to anyone under 1800, unless they really liked that opening for some reason (in which case, I'd say to "go for it" but realize that it will take a lot of study and learning advanced and subtle technicalities). 

By definition, the King's Indian Defense is a hypermodern opening as well, but this one I could make an argument for being a little more manageable and even 1500+ rating (estimate) might be able to learn the basic ideas (like the thematic ...f5 pawn push in this opening) and do fine. IM Levy Rozman (@GothamChess) recommends the King's Indian Defense for beginners because the fianchetto is a setup you can apply against almost anything white does and this is true, but Levy's recommendations for that level are also simplistic and more about getting a decent position rather than the ideal one which requires a ton of work/study.

1. e4 e5 and 1. d4 d5 are common and will give you a taste of everything. It might be tougher to learn at first, but the variety will help you long-term.

I suggest Queen's Gambit Accepted against d4.

If I accidentally ever end up playing 1... d5!? as Black, that would also be my option.

Everything you said is definitely true.
I (U1600 peak) play the Alekhine's (still working on longer lines in Four Pawns and Exchange but I have the basics down) and was considering learning the Grunfeld but was daunted by the sheer number of lines you have to learn.
The KID, on the other hand, is incredibly simple and incredibly easy to transpose into. For a solid month, I consistently played g6 d6 Nf6 Bg7 0-0 c5 against literally everything and then played the position from there. I know e5 is better, but I don't mind the Benoni-esque structure. I also recently picked up the OID for similar reasons, but I don't quite understand the opening yet.
I agree that 1. e4 e5 and 1. d4 d5 will help you get a taste of everything and that the variety will help you develop more quickly (it's the same reason I recommend the Scotch, Ruy Lopez, and Italian to newer players) but I don't the 1. d4 d5 is worth learning. 1. d4 Nf6 gives much more variety, with the NID, BID, OID, KID, QID, Benoni, Benko, Blumenfeld, Budapest, Grunfeld, Neo-Grunfeld, Catalan, London, and every other Indian Game that I'm not listing here. Compare this variety to the QGA, QGD, Slav, Semi-Slav, Tarrasch, Zukertort, Chigorin, and Stonewall, along with the other QPO and QGD variations.
1. d4 Nf6 offers the same amount of variety, if not more, but it's also easier to play and easier to win from Indian Game positions. In the Chess.com database, White scores 58 points against 1... d5 compared to 54.5 points against 1... Nf6, and when AlphaZero taught itself chess, it never played 1. d4 d5 on its own, it always played 1. d4 Nf6.
I wouldn't recommend 1. d4 d5 to beginners, but ironically, I wouldn't recommend 1. d4 Nf6 to beginners either. I'd recommend g6 against anything, considering that no matter whether 1. e4/d4/c4/Nf6 is played, 1... g6 pretty much can always lead to the KID.

@12

"I wouldn't recommend 1. d4 d5 to beginners"

++ I do. It is simplest and best for beginners and world champions alike.
https://www.chessgames.com/perl/chessgame?gid=1937841 

For Black, against e4, I like Petrov's defense.

"In many chess openings, you often find each side has a choice between two or three moves of similar popularity. However, in the Petrov Defense, the main move is likely to get played more frequently by a large margin. In light of this, there is less theory to learn in the Petrov Defense than in many defenses against 1.e4.
  Natural development and simple, easy-to-remember strategies make the Petrov Defense a good choice for players at all playing levels. The Petrov Defense will serve beginners as well as it does world chess champions."

https://www.chessable.com/blog/petrov-defense/

https://www.chess.com/openings/Petrovs-Defense

Even in this game in your post about recommending 1. d4 d5, 1. d4 Nf6 is played. Yes it transposes to a QGD, but that extra tempo of getting the knight out is crucial.
That's why 1. d4 Nf6 2. c4 is so common, because then 2... d5 is a bad move.
In 1. d4 Nf6 2. Nf3 d5 3. c4, however, Black's pawn is already defending, so the trade is worse for White, and Black can control the center easier.
1. d4 Nf6 2. Nf3 d5 3. c4 scores 37/40/23, or 57 points for White
1. d4 d5 2. c4, however, scores 41/37/22, or 59.5 points for White
1. d4 Nf6, last of all, scores 38/33/29, or 54.5 points for White
Note that Black in the Indian Game scores FIVE POINTS HIGHER than in the Queen's Gambit. That's unreal. And if you still want to play Queen's Gambit as Black, this Indian Game transposition still scores 2.5 points better than the regular Queen's Gambit. 
1. d4 d5 is neither the simplest nor is it the most solid. Ironically, its highly theoretical and critical nature would lead me to recommend against it for U1600 players, and 1600+ players would still most likely be better off playing the various Indian Games.