My #1 favorite would be the Alien. Second is predator.
But as for the alien, i like how it requires, in some ways, the most "skill" to be really GOOD at. I have yet to encounter an alien that really goes on a killing rampage when im on the same server. Not bragging (well maybe little
), but i've favorited the alien since AvP 1, and has always been my species to play as online.
And i really notice, that when playing on serves with other skilled players, i really like how they actually start to "respect" the skilled alien players, by not making stupid mistakes as going into tight corridors since they KNOW i will pretty much get them in there. I just like to be the "thing" that everyone else on the server goes: "ah crap there he is!!" Etc...