Awesome, I have a 150g too!
Anyways I asked this same thing before and here are some options.
I'm guessing your talking about freshwater so the list as follows:
FW stingray if the tank width is big enough and not to tall.
Piranhas
Large new world cichlids
Pikes
Arowana
Wolf fish
and lots more, but tell me what you like?
In my 150g tanks I have had 7 red bellied piranhas, and another 150g tank I had 1 oscar and 1 wild red shoulder severum.