This question is almost too hard to answer. I want to go with the nostalgia of Rules Cyclopedia or the 1E DMG but, to be honest, I haven't played either in 10 years or even opened them since 3E came out. I'm going to go with the 3.0 PHB since it actually brought me back to role playing after a loing stint in CCGs and computer games. From the moment I opened it (and being the jaded 'seen it all' kind of guy I was I was fully confident I would hate it) I was hooked again. Since then I have actually been published in an RPG rulebook, ran one D&D 3E campaign to 15th level (something I never did back in the day) , started a second D&D game, ran a superhero d20 game, ran a Dragonstar campaign (still on haiatus but we will return to it one day) and played in a d20 Modern and two 3.5 D&D games. Not bad for a guy who said he would never play or run D&D again huh?
So 3.0 PHB for me (and to a lesser extent the 3.5 PHB)