Because I started gaming in the 90s, the rules I grew up on were AD&D 2nd edition and OD&D 5th version. When 3e came out, I dropped both for all the shiny new d20 system games, but as the d20 system has grown so bloated and cumbersome over the years, it's a change that I really regret.
These days, I'm happy playing my OD&D 5v (Black Box, Rules Cyclopedia, Thunder Rift, etc.), but if I were ever to once again pick up any iteration of Advanced D&D (by which I mean 1e through 4e), it would definitely be 2e. I have so much nostalgic investment in that edition. Baldur's Gate alone brings a warm fuzzy feeling to my heart ("Go for the eyes, Boo, THE EYES!!!"), but the thought of brining those black-cover 2e books back the game table again (along with a copy of the Complete Psionics Handbook and 1e Oriental Adventures, of course -- the game wouldn't be complete without psionicists, monks, and ninjas) does certainly have its appeal. Ah, the good old days...
The special thing about 2e was that it was the edition that came closest to a "heavy roleplaying" version of D&D. It's the version of the game that made city adventures more fun than dungeon crawls. It's probably because I grew up on 2e and the Black Box in tandem that my own play style tends towards such an even mix of method acting and wargamey hack-n-slash.
5v was such a board-gamey rendition of D&D... all the adventures came with little cardboard stand-ups of characters and monsters, and big poster maps with tactical grids for all the rooms. (Anybody remember Escape from Zanzer Tem's Dungeon?) Couple that with the in-depth roleplay encouraged by the 2e Player's Handbook and Dungeon Master's Guide, and you have what is IMHO the perfect RPG.