I would say a payara (Hydrolycus Armatus), but I hear they are VERY, VERY difficult to take care of and you would need at least a 500g for one. Anyway I think you should go with an alligator gar. They look cool, get big, and have lots of teeth.
Hee hee. Dirty idea. Go against the grain and get one of those albino parrot oscar atrocities. Name it something offensive and pretend you hate its guts when other people are around.
Both of these fish get big quick. (Obviously the Dovii gets larger) . They both have big personalities and love to interact with humans. I used to have an RD in a 60 gallon by himself. It was probably the coolest fish keeping experience Ive had. Solo fish are cool either way. Good luck and have fun choosing.