I would say hands down giant snakehead (or other snakehead species...) although i do not know many who keep them... Midas and red devils are very aggressive to (as stated) it depends on what your definition of "aggressive" is. My blood parrot is very "aggressive" in the way he chases me around the room amd chases other fish but he has never actually killed anything. I think that the OP should state what exactly he means when he says "aggressive" do that we can more clearly understand.
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