I almost wish I could pick three out of the whole group. I voted for 3.5, because I've probably never played more D&D with 3.5 than any other rules in the past. And I'm having a blast.
I think Rules Cyclopedia/BECMI plus additions from the Gazetteers could constitute another category, like AD&D 2nd ed. plus Players Options rate a category. As such, adding the Gazetteers with BECMI, I think, was an excellently executed product line. So BECMI nearly rates a vote from me. I wish every setting TSR/WotC put out had done the same.
Lastly, I really enjoyed AD&D 2nd ed. when used in settings that tailored the rules for the setting. I'm particularly referring to the execution with Al'Qadim and Red Steel. My longest lasting DM'd campaign used these settings as inspiration for tailoring the rules to fit the flavor of a setting. It has taken me years to figure out how to do it as well as those settings did with AD&D 2nd ed. ... of course now my old group has all grown apart having gotten married and had children. Since I actually used AD&D 2nd Ed plus some Player's Option options, that nearly rated a vote from me. Someday I'll find a new group around here to game with, until then RPGA game days will do.
So I really have three near equal favorites. Since one will do, I chose 3.5.