My favorite is version 2 with "skills and powers", (essentially 2.5). And, yes, I still know people that play it. Version 2 is complex. I get that. It can be a HUGE obstacle to new players and even more so for new DM's, but that is part of the charm for me. With version 2.5 it was like you had to learn a whole new world in order to be a part of it. With later versions especially after 3, it was more like a world that was there that you pretended to be a part of. One that held you hand throughout the process. You didn't learn what it was like to be an elf. You are TOLD "this is an elf, this is what they do, NO NUANCE, NO EXCEPTIONS!". With all that complexity there was a lot of room for customization, and there wasn't, what I feel, that annoying trend to map out very specific things that your character was going to do or be as he progressed in levels. Characters tended to grow organically and were more real to me. Not only my characters but others' as well.
I had a friend that played a wizard. There were literally tomes of different, obscure spells that he knew or had at least an idea of where to look for a specific spell or desired spell effect. Granted he had to be a hardcore geek in order to keep track of it all, but it lent very well to the idea and persona of a wizard, someone who had to learn and keep track of volumes of obscure spells and effects and at least had the semblance of someone that had spent years studying the arcane arts.