Kelly Freas cover
Ovion from BSG
Speaking of Star Wars, I always found the Quarren race particularly creepy (though I think I'd make mine indigo-skinned to up the "weirdness" aspect).
Skitters from Falling Skies
Selenites from First Men on the Moon
I guess I'm vacillating between squid and insects these days. Man that Freas cover is great though--grotesque, menacing as soldiers but also strange enough to look highly intelligent. Might just have a winner there.