For me, its a tie between Birthright, Al-Quadim, and the original Greyhawk boxed set. Greyhawk is a classic, and can incorporate about anything into it without being over the top. Birthright, because it was so different, and it was the first setting I'd seen where the cultures really came alive and were significantly different from each other. Birthright served as inspiration for my homebrew world after we played in it for a while, and I really enjoyed the political aspects of Birthright as well. Al-quadim was just awesome for its tremendous flavor and incredible settings and adventures.
The only setting I just "don't get" is Eberron. I know a lot of people love it, and more power to them, but to me it just seems "more D&D than D&D." Thats just not what I'm looking for in a game.