Hm. I'd have to go with... Mystara. It always seemed more open to me, with less of a metaplot. That being said, there's a lot to Mystara that is a bit lame - some very campy stuff in it.
I guess I just prefer D&D to AD&D, really, and Mystara has a slight edge. Truth be told, though, if I was going for a generic D&D world, I'd go with Krynn. I'm not a huge DL fan (and I know I just said I don't like metaplot, which is kind of ironic), but I think the world of Krynn is just dripping with good flavour as opposed to the others.
But, yeah, looking at your options, I'd go Mystara.
Wait, Mystara is the D&D "known world", right? As in, Karameikos, Northern Reaches, et al?