The very first game I played was 1st Ed AD&D. However, that doesn't really count, since the DM handled all the rules. (This actually led to my thinking that was how the game was supposed to be, and so up until I went to university some years later, I rolled all the dice for my players.)
The first rules system I actually used was BD&D (Mentzer). Though even that was after a proto-D&D system I cobbled together from reading "What is Dungeons & Dragons"!