Now that I have my megadungeon direction, I have to firmly focus myself on a system. Naturally, it will be old-school D&D, but via which route? For me, it is all about the characters and the combat system. It is about the one that produces characters that I want to play (even though I'll be designing/refereeing). it also has to deliver characters I want to referee, so no abilities that get real sticky with math or exceptions. Combat has to pop, move right along, be exciting, terrifying, and quick. I refuse to track segments or arrow flight times or any of the other "realism" nonsense that people have tried to bolt onto the abstraction engine at the heart of D&D.
So, it really comes down to three options for me: Swords & Wizardry Complete, Labyrinth Lord, and Labyrinth Lord + Advanced Edition Companion.
Complete really is in the driver's seat. The LBBs plus supplements were proto-AD&D, and so is Complete. I think it would require minimal house ruling (mostly for the Fighter, Cleric, and changes to the Magic-User xp chart). Its rules are closest to the ones I started with, so the comfort factor is off the chart.
LL and LL+AEC are superb games. Very well presented and organized. As I've mentioned, I've never held an actual copy of B or X, so there is zero familiarity or nostalgia for me in LL. The AEC does conjure some memories, though, because it truly does capture the spirit of how we actually played "AD&D" back then. I just bolted it onto the LBBs instead of B/X. The biggest thing holding these two back, beyond the nostalgia factor, is size. I have a program that splits pdfs into their individual pages, then add or rearrange pages. I cobbled together an LL "complete", which is base LL minus sections 2 and 3, added in everything from the AEC, left out the known world stuff, and it is still 236 pages. Not bad compared to close to 500 for Pathfinder Core, which doesn't include monsters. But S&W Complete comes in over 100 pages shorter. I like that.
The other option I didn't include is my own S&W based house rules doc. I still have mixed feelings on it, so it will almost certainly remain on the shelf for now.