if i could have any fish in the world it would be a tank of Mola mola there are such awesome fish.
Heres a little description of them
The ocean sunfish (mola mola) is the largest bony fish in the world. It is a unique pelagic fish, and specimens of ocean sunfish have been observed up to 3.3 m (11 ft) in length and weighing up to 2,300 kg (5,100 lb).