I doubt I'd ever get back to it, since I prefer more recent editions and I've also been working on my own game, but 2e used to be my main system back in the day and the first game I ever DMed/GMed (I started out with Basic, but played it only as a noob player). The Complete X Handbook series was perhaps the greatest series of supplements in D&D, IMO. Hard to single one out as the best, since they were all good, and it's been too many years to recall all the details, but it kinda depends on what class or race you want to focus on.
The Complete Cleric's Handbook did provide a lot of material for crafting Specialty Priests, though—which can help expand the class significantly—so that one's probably a good one to start with, but they were ALL good. Might want to get the Complete Spacefarer's Handbook as well, if you grab the old Spelljammer setting boxed set. It adds a bunch of spacefaring races, and kits as well if I recall correctly.
The Player's Options stuff is a bit take it or leave it IMO. I did play around with it a bit, but they add massive changes to the rules that are a bit fiddly and poorly balanced, if I recall correctly. But they add a bunch of customizability to the game.
The best 2e stuff other than the Complete X Handbooks are the settings, IMO. Though, those depend a lot on what your preferences are. Dark Sun is by far the most original D&D setting, but I recommend going with the original boxed set if you're getting it, cuz the Revised one kinda sucks, and they made a lot of changes to the setting that altered the tone around that era. Earlier supplements for it are the best, particularly stuff like Veiled Alliance, Elves of Athas and Dune Trader.
Ravenloft was also pretty good setting, if you're into gothic horror, and I regret never getting it (I only got access to the books from a friend who got most of them, but I lost touch with him over the years). Planescape was probably my favorite setting after Dark Sun, but I never got many of the supplements, so can't recommended any. I also played a lot of Spelljammer, but I only recall the Complete Spacefarer's Handbook, which I already mentioned.
EDIT/PS: You'll also want the Complete Psionicists Handbook if you get Dark Sun, since the setting makes heavy use of psionics, and relies on it a lot. Unfortunately, that one's actually the worst book in the Complete X series, but not so much for the quality of the writing, but because the class was poorly implemented. But you are going to need it to really get deep into Dark Sun.