Birthright. Most definitely. It's the strongest world D&D ever had, although Eberron hasn't had enough momentum. Dark Sun? Probably, as long as it was updated correctly.
Though I have never played in it, I have always been interested in Dark Sun so I would surely get that. It is really the only one listed I would be interested in. But I might get the others just for my collection
I voted Spelljammer and "Other" (Planescape and Al-Qadim), but I'll be honest with you - right now I'd likely buy nothing.
Now, if this came up 2 years ago, I'd be all for buying updates. But nowadays? No. By this time, I've updated everything I've needed to update (except for the Complete Sha'ir's Handbook. I've still got to look at that one).
Planescape. I'd pimp my players to sailors on shore leave to get it to happen.
(Dude you forgot to include Planescpape, for shame... j/k... mostly)
And while I never got to play it since it was cancelled an edition before I started playing DnD, I'd probably get a Dark Sun book just out of curiousity.
I'm a huge Planescape fan, but I think there's plenty of planar stuff built right into the core of D&D that a "one-shot" wouldn't be all that necessary. But then again, I own all of those lovely 2E boxed sets if I ever need the "fluff".