It's amazing how much has changed in the world of the OSR since this thread was started. There are no longer any "out of print" editions of D&D, a very wise move by WotC, which only solidified their market share after crushing it with 5e.
While I LOVE Labyrinth Lord, I usually run either B/X or the RC for older style play. Perhaps the most intriguing retro clone that I've come across recently, however, is Untold Adventures. It's a simplified version of 0e D&D. That's right, even more simplified! It has a few very elegant systems in place which are very cool. I could see running a great swords & sorcery game using UA.