If I had to pick a campaign world that I've used in my campaigns - in other words, a world I've actually experienced at the game table - I would have to say the Forgotten Realms. Yes, warts, baggage, and all. Say what you will about the Realms, but it's got a lot of good stuff to it, a richness and depth from decades of existence. I know, Drizzt is a Mary Sue, the Time of Troubles was a bit ridiculous, etc. But I love the Realms in all it's iterations.
Well, perhaps the recent Spellplague iteration is where I draw the line.
But, I'm hoping for some Realms "redemption" with the new Sundering "reboot" coming out in the (near?) future.
I never owned the legendary "Grey Box" version, but when I ran my recent Realms campaign I used the 3E version. And I consider that campaign to be my most successful effort, at least when it comes to my return to the table-top within the last few years.
However...all that said, I have some of the old TSR materials for Fritz Leiber's Nehwon that I've been dying to use for the longest time! It is very intriguing to me when I think about running a campaign set in the city of Lankhmar and the world beyond! That's definitely on my RPG bucket list.