Favorite System
I've played a variety of systems over the years (BRP in its numerous incarnations, D&D, AD&D, Traveller [the original], Warhammer FRP [again, the original], ICE's system whatever that was called, etc.). I'd have to say the current D&D is probably my favorite, but only because they've mined all of the other systems for ideas and game mechanics, taken the best of them, and incorporated them into the 3.x rules (damage reduction from WFRP, skills from BRP, etc.).
Actually, come to think of it, some of the recent OGL standalone RPGs derived from D&D are my favorites, especially A Game of Thrones, which in my opinion is the ultimate evolution of the d20 system

. Too bad we'll likely never see any followup products for it, sigh.
I still really like AD&D (1E) too, and if it were the only thing around I'd still be playing it. I played it from around 1980 to 2000 then switched to 3rd edition, skipping 2E entirely.