This is a damn tough question for me.
I'm running an AD&D 1e / OSRIC Campaign, but I've houseruled the crap out of it. Without houserules, I don't think I could stomach AD&D, as much as I identify with it.
By the book I think I'd have to default to Labyrinth Lord with Advanced Edition Characters, although if I were restricted to one book I'd go Swords & Wizardry Complete - the single save does have it's advantages, and it gives me most of the AD&D characters in a single book.
I default to the clones over the originals as they are generally less confusing rules wise. I'm sure others will default to the unadulterated originals, but my problems would be we never played AD&D as it was written.
So, what would you chose as you BTB system?