Basic Fantasy Role Playing, that is. It's sort of the forgotten red-headed step-child of the retro-clones. Being a red-head myself, that makes me partial to it.
Before Labyrinth Lord came and stole the BX clone thunder, Chris Gonnerman had created this little gem of a game. Unlike LL, every game rule, module or supplement for it is available for free, and print versions are available at cost. It also borrows some of the tropes and mechanics of both AD&D and 3E D&D. Race and class are separate. Ascending AC and attack bonuses. 20 level classes.
But what it keeps are the simple rules for explorations and combats of the Classic D&D line.
Personally, I would use BFRPG over LL/AEC if I wanted to have AD&D style race/class separation. I'm fond of the racial classes of Classic D&D, but they don't always fit the mood of the game I want to play at the moment (whatever moment that may be). Plus, I'm a fan of ascending AC as it's one less thing I need to worry about as a DM.
So if you're thinking about running a new game, why not download the free BFRPG rules, give them a look, and consider using them for your next game? I've pretty much decided that when I get back to DMing my Megadungeon, they will be my go-to rules set.