any kind of piranha. and i believe you cant have gars either.
you could go with arowana like the jardini mentioned above or silver arowana. those are about the only ones you can get. you can probably find an african arowana locally, but you wont like it. and maybe you can find a black arowana, but its gonna be very expensive
If I were to set up a single fish tank, I'd get a Jardini or a Giant Gourami (or Red Tail Gourami). The Gournami's are ugly, but they are like puppies. Tons of personality.