I voted for Hiver and Droyne, because they seem the most "alien" and therefor original. I mean, you see cat-girls and dog-boys all over the place. Humaniti is just "us" with different cultures, and the K'kree, well, I don't think I need to expound upon that.
So it's really the neat sense of "new" that comes with those two races. I try to include them in any game I can. They're always a hoot because properly played, the PCs seldom know what to expect of them. I once did a wicked horror-style game where the PCs come across a small research station where everyone is dead. The only survivor is a droyne scientist (technician) that tells them that there is some kind of invisible energy-based alien that killed everyone else. The only reason he has survived is because he managed to build a handheld detector for them. He gives the PCs one while he keeps the other. Then I have the odd NPC wander off alone and hence gets killed. Then I leave little hints like their detector never goes off when the droyne isn't with them, only when he is, and that the only time people die is when the droyne isn't around. The truth in the end was that the technician had gone psycho, and was killing people while invisible. He could manipulate both "detectors" that really detected nothing, and were just a ruse to manipulate the PCs. They eventually figured it out, but not before I had them both scared and confused for a good while! One PC ended up becoming racist against droyne because of that incident. :devil:
Within the human cultures though, I do have this inexplicable leaning to the Solomani. I mean, I don't like supremists normally, not even to role-play. But somehow with them being from Earth, and the other humans not so much, and those damn Impies always trying to lord over us just because they discovered jump drive like 1000 years before us, and then steal the cradle of humanity away from us...
Sorry. Maybe it's just fun to play that extreme once in a while. Maybe it's a lesson on how easy it can be to fall into that mindset...
o: