LOVE the expression on six! Although he's alien and that look may mean something entirely different he looks rather like the intelligent guy giving an 'i can't believe you just said that stupid thing' glare.
My favourites are #5 and #9. 5 I imagine to be intelligent, blue-skinned race and I like how he (or she) has human-like features, but still looks... well, alien. 9, on the other hand, seems like the type whose bad side you don't wanna get on. Such badassery! btw he looks sort of like a mix of predator and alien from Aliens vs. Predator.