Hmmm...favorite aliens...you mean to meet, or to serve as meat? I do hear that some Solomani of Scottish descent have a fine recipie for K'Kree haggis.
Okay, I know...this post isn't about old episodes of Iron Shugilli.
Anyway, I'd have to say, in no particular order, Vargr, Bwapp, Aslan and Darrians, with the Virushi getting at least an honorable mention, perhaps the Hivers also. Since I've had T20 I've found myself generating a fair amount of Luriani also...including one that started out as a Belter (checked, their homeworld has a planetoid belt) that ended up in the Scouts after both narrowly making a survival role and not being able to "reenlist" as a Belter.
As an aside, I'm trying to picture what a Luriani Belter Seeker would look like...considering they are resistant to cold and are aquatic, I keep on picturing the ship being full of ice water...
Okay, I'll stop rambling, I've already answered this question a couple paragraphs ago. Even if, as Emperor Cleon the IV it's my perogative to ramble.