there are a few species that are known for being absurdly aggressive. The problem with the fun ones is that you need a HUGE tank and accomodations, and limited or preferably no tankmates.
Though boring and fairly unintersting looking, Wallago attu (aka Man Eating Catfish) is a voracious predator with a ridiculous set of chompers. They generally lack the ability to put on a continuously entertaining show aside from feeding times. I would advise keeping one of these in a thick acylic tank in a quiet area because they are well known to bust through glass, especially in small quarters.
From what I hear, the Aba Aba shares similar characteristics but lacks the "Man eater" label. Either fish would require an immense solo setup at maturity, but I thoroughly enjoyed my experience keeping W. attu.
Also, there are very few (if any) vicious sharks that can be kept in the confines of even the largest home aquarium. You could keep a small true marine shark but feeding would not be up to your expectations. Stick to the badass freshwater and check out W. attu, you'd be amazed at what it will eat.