What is your favourite alien from literature, motion picture or computer games?
The Times (UK) have listed their The 40 most memorable aliens here.
The problem of most aliens is that humans have a tendency to anthropomorphise. So most fictional aliens aren't terribly alien. We have a tendency to see such as merely friendly or hostile, without considering the alternative: alien.
Star Control 2 had some of the most imaginative aliens, but my favourite alien happens to be Orz. The Orz make you question whether your whole reality is being raped.