I voted for Greyhawk without any hesitation. It is the only campaign setting I played in until the coming of Eberron. I bought one FR product, Doom of Daggerdale, which I converted to fit into my Greyhawk campaign.
I'd buy none of the campaign settings the wizard just has sitting in their pockets from the days of tsr. I'm a big planescape fan, but I don't feel that one book could do it justice. All the intresting places and situations are what makes planescape intresting, and if you look at the orginal box set, it was quite lacking in describing the planes when compared to the later box sets and monster compendiums that held the real magic.
Don't get me wrong I would love to see planescape in print again I just want to see it done right.