I like the Mellor, though that may be more of a weakness on my part for shapeshifters. Though, even with that aside, the idea of a race who was meant to be an arbiter of peace and prosperity, corrupted into something horrible is something I find thematically interesting.
Mellor aside, I think the Dimixi come up second as most interesting. As improbable as a human-like society may be for spider-like things, it's also kind of an interesting subversion to have a bug race not function as a hive. It's almost comforting after a fashion to think that you could run into something out there that may look quite different form you, but still has some similarities when you get past base appearances. And besides, hey, they're spider people. Spider people are cool.