I clearly havn't seen enough space monster action!
If we are going with the definition of "scary things which go rawr and are in space" I am rather partial to the Reavers in Firefly, though they are arguably not alien monsters at all. The fact that they were once men is the subject of some moral ambugiuty, as well as Kaylee's musing over what made them get that way. They also bring a large amount of scary with minimal prosthetics/cg.
If we are going with the "completely alien species" sort of monster though, they don't apply, and it's said that Gina Torres only agreed to do FIrefly because of the lack of cheesey monsters.
I do have to agree with Candy that Star Trek has some great ones, but also didn't assume they were the enemy as soon as look at them. Man that show is the greatest
You've got to change the world,
and use this chance to be heard.
But dancing is fine too