I would get two Manganese (aka Jaguar) Cichlid, I have a friend with 2 in a 30 gal, I don't condone this he is cruel. But they are the meanest freshwater fish I have ever seen and they are readily available at most pet stores. I watched them shred a 7" catfish because he was in the tank. They are very protective of their young, if you hold your hand over the tank the male will jump out of the tank to attack your hand. I understand the Wolf Cichlid is much the same. I personally would keep Oscars but you said you want aggressive. Here is a you-tube video of a Jaguar Cichlid, and they do get pretty big.
http://www.youtube.com/watch?v=xGYvfrQy2_M