A few thoughts on each one:
Al-Qadim - I'd like to see more done with this one from an interest standpoint, but I rather doubt I'd buy it new.
Birthright - while I'd love to see this one re-done and would probably even buy it, I just can't see the Birthright setting being compatible with 4e's Points-of-Light design parameters.
Dark Sun - heard of it but never seen it (though it seems to have some fans here!)
Dragonlance - works well as a series of books, somewhat well as a series of adventures, but does nothing for me as an overall setting. I'll pass.
Greyhawk - having played in it, I can't say it does anything for me...though it has some nicely stealable deities for insertion into other settings...
Jakandor - never heard of it.
Kara-Tur - barely heard of it.
Maztica - again, from an interest standpoint I'd want to see what could be done with this, but I probably wouldn't buy it.
Mystara - sign me up. Now. If I was ever going to use a pre-conned setting again, Mystara would probably be it.
Planescape - I'm not a big fan, though doing something with this might force a rethink of the rather bizarre (and minimal) plane system 4e has shown us so far; and that alone would make it worthwhile.
Ravenloft - I always thought this was a setting with a ton of potential that was never quite realized. It deserves another chance, but I'd want to see what was done with it before deciding to buy or not.
Spelljammer - though this setting has some fans, I am not one. Sink it.
So, I guess my list would go something like:
Mystara
Birthright (if they could find a way to make it work with PoL)
Ravenloft
(Al-Qadim - for interest only)
(Maztica - for interest only)
After that, I don't care very much.
Lanefan