Precis Intermedia (pigames.net). Home of Shatterzone, MasterBook, Warcosm, Bloodshadows, Supergame, Ancient Odysseys, genreDiversion, Story Engine, Active Exploits Diceless Roleplaying, Disposable Heroes Paper Minis, Two-Fisted Tales RPG, and more.
If you strictly mean an OSR D&D style game? Depends on the mood and game play desired-
S&W Whitebox when I want to make my own detailed house-ruled variant
Iron Falcon when I want to adhere as closely as possible to my LBB days
But for an edition of D&D? Moldvay/Cook/Marsh B/X sets.