All good campaign settings, but you just cannot beat the classics. The world of Greyhawk was the first campaign setting to be fully fleshed out (Blackmoor wasn't half as complex), and was the brainchild of Gary E. Gygax, founder of D&D. The setting was like FR in the fact that it had every terrain feature, plenty of deities, allowed many options, and was a very classic Tolkien-style setting. It differed (and was better, in my opinion) by being much more gritty than the sometimes flowery Forgotten Realms.
Of course, anyone who reads my name will realize I'm sorta biased on this.
Runner-ups would be Dark Sun, Forgotten Realms and Ravenloft.