I'd have to say AD&D, as far as published "OSR" rules sets go. OSRIC is very solid. I also like Astonishing Swordsmen & Swordsmen of Hyperborea, although I like it more for the subclasses and setting than for the rules themselves. I always run games using the 1e books. But I sometimes draw on OSRIC or AS&SH supplements and modules. And if there are players that want a copy of the rules, I'm happy to point them towards the free OSRIC pdf.
I also like original D&D, of course, but again I favor using the original TSR rules and then modifying them to suit your game. That's the biggest strength of original D&D, in my opinion. Consequently, I don't draw on the original D&D clones much, although I'd be happy to use a S&W module (for example) in my original D&D game.
I think Lion & Dragon is pretty cool. I haven't run it as its own thing, but I've drawn on it as a resource for certain original D&D "mini campaigns" that I've run.
I guess that's my pattern: I don't really run "OSR" games, but I love being able to draw on OSR game supplements/adventures/etc. as resources for my 1e AD&D and original D&D games.