I like I6 Ravenloft and it's sequel I10 (I think it's I10) Ravenloft 2: House on Gryphon Hill, I am planning on adapting them to use on my 3e players
D&D Adventure modules - B2 The Keep on the Borderlands

I'm a 5e DM, currently running through the Phandelver adventure module. Its not bad. Not great for first thing to go through I've heard, but this isn't our first rodeo x3

Apparently 5e is a lot more like 3.5e, which can only be a good thing - 4e was much more like a card game and (for mine) missing some of the quintessential D&D flavour.
I've only ever heard about Ravenloft - I was going to post all of my favourites in this one thread, but then I thought it would be good to post individual modules in individual threads (or at least in a series) so as to prevent spoilers for people who might want to play them some time in the future.
I actually started work on a 3e conversion of Keep on the Borderlands with a guy from somewhere in Europe: never got it finished, but it was a fun experience.

I found the same thing with 4e. I actually have not tried 3.5, just 3. And converting AD&D to 3e is easy as long as you don't worry about every detail. All I usually do are AC, attack bonus, saves, checks, and maybe some special stuff.

3.5 is actually just a minor tweak to 3rd edition in ways that made sense: from memory, there was a change to the Damage Reduction rules - it's easy to find a summary online, like https://rpg.stackexchange.com/questions/69772/what-are-the-major-differences-between-dd-3-0-and-dd-3-5

One of my friends plays 3.5, but I have memorized almost all of the 3 rules, and know where to find everything, so I stay with 3.

From what I hear 5th edition is incredibly simple compared to most other RPGs, including 3.5 and Pathfinder, making it great to start off with, but frustrating to experienced players who enjoy complexity and customization. Additionally, the rules are quite vague, leaving a lot up to the DM to judgment call. But, I like it. I'd be happy playing other systems, but I wouldn't wanna DM them! (Pathfinder 2, currently in playtest, looks pretty cool).

I haven't triad Pathfinder 1, so I didn't even know that there was a 2. I change a lot to fit my players' (and my) tastes, so a lot of DM calls are made anyway when I play.

Can't say I've done any checking, but I thought Pathfinder 1 was 3.5e rebranded.
Anyways, the Mad Hermit in the woods was always fun and the lizard men in the swamp deadly, but the real action was always in the Caves of Chaos. I think the gnolls were the hardest but the temple was the scariest, but there were so many cool elements. Two tribes of goblins, the hobgoblins lording it over both of them, the kobolds setting traps, the gelatinous cube and the owlbear: classic D&D monsters.
I also really liked the way the writers made you think about the answer to what happens if you go away from the caves only having dealt with some of its inhabitants (and let's face it, you almost need one expedition to deal with each cave, especially when starting at 1st level) - do other inhabitants take over empty complexes? How do they prepare for further incursions? It was invaluable training as a DM, and less intimidating than the blank spaces.
I've always been a fan of adventure modules - I've always been the DM, and constructing completely new worlds a la the Malazan Book of the Fallen has never been my thing: I'd much rather fit something together into an overall arc. There will be spoilers in these posts!
The red box D&D Basic set was my first ever D&D
I loved Jeff Dee's art in particular. B1 In Search of the Unknown had some massive blank spaces for the DM to fill in, and I have to admit that didn't appeal to me. B2 The Keep on Borderlands left the keep itself a bit light on in terms of details, but the caves themselves were complete and I loved it.
Will post some more about it later.