I like the Xindi. The idea of 6 separate sentient species evolving on one planet fascinates me. I think they seem like good Federation material since they have proven they can overcome their differences and present a united front.
My favorite are the Aquatics. They remind me of our dolphins here on Earth. They are considered to be highly intelligent, but we haven't yet learned to communicate with them. The Xindi are very advanced for having learned each others' vastly different languages and adapted to each others' needs.
I wish we could have met the Avians.