First off, all the various retro-clones are pretty similar - they work off different editions that were themselves quite similar.
For simplicity, I'd say you're looking at one of three: Swords & Wizardry, Labyrinth Lord, and BFRPG. The latter two BOTH work from Moldvay Basic; LL is a very close clone, and BFRPG inserts some more modern mechanisms, plus tweaks.
I would just read them (each has a free pdf), and see what kind of vibe you get. Each one is supported with various modules, and each one has a messageboard to visit. The one for Swords & Wizardry is at
Swords & Wizardry Forums • Index page - I can't remember if BFRPG has a specific forum (there is definitely a website) - check in the simulacrum games forum over at Dragonsfoot...
Then think to yourself whether the game supports a "campaign feel" or if you'd feel limited by it. Swords & Wizardry lets you (or forces you, depending on the viewpoint) to do some imagining about the campaign. Labyrinth Lord tends to provide you with a bit more crunch/guidance in that regard (IMO). I don't remember well enough to say where BFRPG comes down in that area.
Most of the modules for any of those three games are highly compatible, so you aren't foreclosing many options with the choice of rules.
I recommend Swords & Wizardry (but I wrote it, so apply the "grain of salt" test on that). We'd be glad to see you at the forums, if you want to stop by!
Matt