I'm voting Eberron.
Of those settings, it's the one that currently appeals to me most.
I'm fond of Dark Sun, though, and Greyhawk would be cool.
Dragonlance....I just...eh. I'm playing a DL game right now and it's not a setting I love. I don't hate it, but there's too many one-dimensional character types (Kender 1: "Hi, I'm a Kender, my defining trait is that I'm childlike!"; Kender 2: "I, too, am childlike and filled with wonder!"; Kender 3: "Unlike those other two, I am very childlike!"), too closely tied with the One Big Story to have much diversity, blah blah blah.
We also had Tyranny of Dragons which is close kindred in high-level plotline.
Greyhawk has a Moorcrockian Low Fantasy edge that I can get into as a counterpoint to the shiny happy Forgotten Realms, and Dark Sun is pretty much always welcome, but Eberron is more my interest.
....kinda want to do an Eberron game, actually...