If by mega-dungeon the first thought is the room-by-room clearing of a massive complex that seems to have little rhyme or reason then I can see why folks want to avoid those.
If the first thought for mega-dungeon is more of the microcosm of life in a large subterranean world then I tend to enjoy those.
I voted for S&W as it is a very light and fast-play edition; not too many rules to get in your way, and th flavor is perfect for an old-school classic dungeon crawl.
Alternatively, I'd suggest BFRPG, which, in my experience, is extremely easy to run and even easier to prep for. It plays like old-school D&D but has a few touches of new-school comfort in it such as the separation of races from classes and ascending AC.
Both are extremely easy and quick to make characters for, which is a good thing if you want old-school megadungeon lethality.
I love 1E for the flavor and the fiddly bits, but don't like all the little crunchy nuggets in the combat system. I'm kind of leaning toward running it with the RC, but allowing players to use the 1E PHB and having the 1E DMG on hand for reference/inspiration. But that basically says build it in Basic/Labrynth Lord. I only plan on running it intermittently, at mini-cons and game days and so on, so I want it to be quick to pick up and play.
Definitely use the 1e DMG as a resource, no matter what edition you end up selecting.