I'm very fond of the Xindi; all of them, really. I liked the strange nobility represented in the Xindi Council; how these five races clearly can't stand each other (well, Arboreal/Primate and Reptilian/Insectoid seemed friendly enough, but beyond that...) and yet they remained truly committed to the idea of the Xindi as a unified people. After the homeworld was lost they could have fragmented, but they built the council instead. And throughout Season Three, there was a sense that they were trying to make it work. They're like a five-way racial feud sitting around pretending to be an alliance, and they no doubt have a regularly changing "[...] days without civil war" sign on the council door, but despite all this they're still in it together. That undercurrent of loyalty was definitely their most striking feature, and did a lot to convince me that they indeed had potential to one day join the Federation. And that, when the Xindi-Primate councillor said he thought the Reptilians would come back to the council eventually, he was right.
I think the Arboreals and Aquatics were my favourites; the slower-paced, more contemplative Xindi. Perhaps because Xindi were introduced as a military threat, and it was a good move to have two of the races seem far removed from the Proud Warrior Race trope; reinforcing the sense that it's a complicated situation, not just "aggressive race attacks".
It's a shame we never learnt very much about Xindi culture(s). From what we did get, the most interesting bits relate to how the Xindi races view one another. There's a good bit in one of the novelizations where a Primate has a string of racist observations: "The Reptilians thought with their brainstems, the Insectoids didn't think at all, and the Arboreals...they thought, all right, they thought about napping upside-down in trees".