The Aliens that you would least like to run into. Chances are if you go to another planet you will not see these guys, but if you do you're screwed. Or the alien is just a jerk and would be annoying to talk to.
http://www.ranker.com/list/what-fictional-alien-would-you-least-like-to-encounter-/stokified,
Worf
Marvin the Martian
Kang and Kodos
Alien Referred to As Alien
Zerg
Critters
Protoss
Urdnot Rex
Reapers
Pitch Black Creatures
