Species 8472 (Star Trek: Voyager)
Klingons (Star Trek The Next Generation and onwards)
The Borg (Star TreK: The Next Generation and onwards)
The Weeping Angels (Doctor Who)
How many species are you expecting to allow in? Everything? The top 64? 32?
I'll nominate my fifth after some more thought!
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The batman villians all seem to be one big joke that batman refuses to laugh at - SantaRPG