I would say:
-Planescape: My absolute favourite. I understand that some people despise the metasetting element, and I actually agree when it is forced upon pre-established settings. However, Planescape can be played as itself, not as the "thing stuck between this world and the other".
-Dark Sun: I always liked the post-apocalyptic & magic style of this setting, which I played not as much as I would have liked.
-Ravenloft: Not quite sure why, but this setting has always been interesting for me. Perhaps the fact that one of the best campaigns I remember playing was set in the Demiplane of Dread has something to do with it.