I voted "Birthright".
Probably I would buy the FR setting book, which is the most likely to be published, but at the same time I feel like FR has been screwedup forever after so many edition changes, so I am not holding my breath to it, I don't expect the setting to be exciting anymore.
I never liked Eberron, and while I technically played Greyhawk adventures, I never really found anything special about it to justify buying settings books. Dragonlance also never attracted me much.
A new Planescape book would be great, but it would be even better if they could reprint the old ones.
That leaves Dark Sun, Birthright and (off-the-poll) Al-Qadim. Probably the latter is what I would like most, if it got a serious design effort and resulted in a single large (300+ pages) sourcebook. But since it's not in the list, and Dark Sun already recently received attention, I decided to back Birthright, even tho I think it's very unlikely.