Hands down my number one all time favorite is 1st and 2nd edition Krynn. 2nd favorite is 2nd and 3/3.5e Forgotten Realms. I have to admit, though, that Pathfinder’s Golarion is starting to grow on me.
I haven’t delved too far into the 5th ed changes for the Forgotten Realms. After all the stupid crap they did in the earlier edition, I haven’t found myself all that interested in looking into it.