I'd say Forgotten Realms without question, usually, but in this case I'm not so sure. I'm not at all happy about the proposed changes to the Realms in 4E. And this coming from a guy that absolutely loves everything he's heard about the 4E rules changes. Unfortunately, they're hammering the Forgotten Realms with the fist of an angry god in order to explain away the changes, and IMO, those changes are overkill.
Aside from that, I'm really hoping someone will put out a Ravenloft setting. Hopefully WotC will license it out to Arthaus, again, as they did a pretty good job with it. And besides, even if WotC were interested in bringing it back on their own, they've got this whole "One setting per year" schtick which would hold them back. Meaning that the absolute earliest they can put out a Ravenloft Campaign Setting book is 2010 (2008 for the Realms, and 2009 for Eberron). So however good a job they could possibly do, I'd rather Arthaus had the license, as they'd probably do it nearly as good, and sooner. But whether Ravenloft gets a setting book or not, I'll probably see about looking for a Ravenloft game not long after 4E comes out. I'm willing to convert it, myself, it it means I can play it.