I voted for Eberron.
I got into D&D in 5th grade. We all had uber kill anything munchkin characters of course.
Between then and high school, I got much more interested in the roleplaying element of things, and eventually got fed-up with D&D as a whole. Too formulaic, too constrictive, too much "been there, done that". And despite having a couple of the Dragonlance and Forgotten Realms novels, I realized I really didn't care for the settings... They just seemed very artificial, put together for the sole purpose of making it a viable lifestyle to wander around the countryside robbing goblins of their money and treasure.
3rd Edition came out, and I was well into my twenties. I was long done with D&D, having moved on to GURPS, champions, various White Wolf games, Exalted in particular. Still, I bought the Player's Handbook for nostalgia and to see what they'd done with the system. I liked the changes, but I still wasn't going to play in those tired old settings.
When I read Eberron that all changed. It's a real world. Not just a set of artificial conflicts. It breathes, it has real politics based on real pressures. A real economy... You even have the option of your characters being in someone's employ and earning paychecks in a realistic manner, not merely subsisting off scavenged loot.
And it's so different! The magical "technology" changes the world so much, and makes it make so much more sense! Besides, the appeal of airships, the lightning rail, warforged, artificers, and everything else along those lines is just inescapable. Traditional medieval european themed fantasy was something I was tired of. Especially since the old settings were in my opinion totally unrealistic, bizarrely warped reflections of it.
I love basically everything about Eberron. The fact that the gods don't intervene in things, the fluid alignments, the ability to delve into shades of grey where the true roleplaying lies, in my opinion... I've always been a fan of psionics as well, so I liked their inclusion. I'd rather like to see Incarnum given an explicit integration to the setting as well, but we can't get everything we want.
When I run campaigns in the setting I don't have to change much at all, and I've been a MAJOR homebrew type GM, going so far as to create my own whole system and setting at one point (Which was strikingly similar to Exalted, I kind of had a bit of deja-vu when that came out), so me taking a setting and leaving it mostly alone is a compliment! But yeah, aside from making the magical "tech" a bit more pervasive and developed, I leave the setting remarkably untouched.