I don't think I'd be the kind of person to take other's advice on what to keep in my gaming collection, personally.
Well, the thing is, they're all tied for the #1 spot in my brain. Conceptually, I think I like Ravenloft best, but the execution has never done much for me. From a practicality standpoint, Greyhawk probably wins out as I own every pre-wars supplement and module for it. Birthright is simply something special that really tread new ground for AD&D (and, frankly, I don't think any official D&D campaign since been quite as innovative).
As for the question, keep them all. None of them are coming back, most likely, which means you will not be able to get materials for any reasonable price.
Keeping them all is what I'd like to do, but it's not really in the cards. I have this new-ish rule about not keeping stuff I haven't used in the past year - and normally, that would mean they
all go. I'm making an exception to that rule this time, though, as the editions of D&D that I play don't come with a built-in setting, so I need to keep one of them handy.
Incidentally, after weighing my options and the opinions of others, Greyhawk won out - partly because it's the one setting of those three that is easiest for me to find players for, partly because I own tons of it, and partly because of the opinion poll.