7. Andy Serkis
There are few actors nowadays who disappear into their characters as completely as Mr. Serkis. He disappears so thoroughly into his characters, most casual fans of his movies don't even know what he looks like! This is probably because his most famous characters are CGI, but his dedication on the motion capture stage proves that he has the right physical commitment to completely embody an alien character.
What Sort of Villain Would he Be? It is too easy to cast Andy Serkis as an alien, but if Gollum is any indication, that is the role he would fit like a glove - most likely a scheming one who works with the main characters but always has his own agenda on his mind. Maybe that might be a little too reminiscent of Gollum, so let me suggest another alternative: physically unimposing but powerful and influential gangster. That is a role he could embody both as an alien via motion capture, or as a human, because yes, he does act outside of the green wetsuit on occasion.