Piranhas are not the nasty predators you think they are. Well reds arent. You say you have a 55 Gallon what are its dimensions? Depth plays a part in this. How long have you kept fish? Have you an idea what you want? Coming from the U.K that puts snakeheads on your list you cant get much more of a predator for your money there. Just start of with some Dwarf Snakeheads, if you are new they are bullet proof and nice to look at. Have a look at.....
Wolf fish are cool very nasty but dont do a lot in the day had a few also nearly had my finger so certainly get the nasty score. Once again dont play well with others. The problem being if you want a really nasty fish you need a bigger tank. The wolf would be ok in that as not very active Have a look at this. http://www.thetropicaltank.co.uk/Tanks/mytank11.htm
I have had a wolf fish. They are so freaking boring. I had him with several larger cichlids and I would wake up every morning and a fish would be missing or dead. He basically sat there all day and did not move though. My exodon's are more even more fun....toss a goldfish in and watch the punishment.