Disclaimer: My opinions only. Worth nothing more than the electrons they're printed with.
The Forgotten Realms: highly detailed (possibly too much for some), high magic blow-stuff-up-good feeling to it, supplements out the wazoo. I don't enjoy the "there's always someone tougher behind every tree" feel, but others do.
Greyhawk: If you loved it before, it's sitting right there for you to breathe life into again. And if you don't do it, nobody will.
Dragonlance: No fun to play in someone else's sandbox that has all the sandcastles built for you ahead of time and the sand removed so you don't hurt yourself by accidentally eating some.
The Scarred Lands: Flavour flavour flavour. Wierdness you may or may not like. Some balance issues, but not critical. Plot hooks just jump out of every nook & cranny. Unfortunately you have to constantly listen to Nightfall telling you how great it is.

Lots of addons to read.
Kingdoms of Kalamar: Absolutely gorgeous atlas. Nice historical background. As flavorful as a mouth full of sawdust, but maybe that's been enhanced with later add-on products I do not yet own.
It all depends on what you're wanting. I think that Kalamar is the closest to old Greyhawk type blank-slate worldbuilding with hints of greater things lying under the surface, but I'll be damned if I can pick up on clear mental images of any of those things in my few aborted attempts to plow through the books. Maybe I'm just getting old, but the KoK books put me to sleep every time I try to make a concerted effort to plow through them. I desperately want to like it... I just haven't twigged yet to whatever breathes life into Kalamar.
Ultimately I yearn for what IKCS promises to deliver, and am interested in what Eberron might surprise us with.