All clownfish are in the Damsel family and exhibit similar traits. The most aggressive is without a doubt the Maroon. I would rate the Tomato and Clarki about evenly in second place. If you are looking for a more docile fish I would suggest either the skunk or the Percula or Occelarris.
To give you an idea of agressiveness, when I worked in a pet store many years ago we got a shipment of fish in and put a small volitan lionfish in the same tank as a large Maroon clown. When we came back the next day we found the lionfish and clown dead. The Maroon had tried to eat the lion and wound up with the lions first dorsal spine projecting through the top of it's head. The maroon killed the lion with one bite and died for his troubles. pretty good proof that clowns can get downright nasty.
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