Lots of good answers so far. I think a short-body flowerhown would be a good choice if you want to go the single larger fish route. A 75 is the minimum I would put a normal FH in, and depending on the fish it may outgrow that eventually. Oscars and big bruisers etc, I would forget about in a 55. 75 even will be a bit too small for life.
If I were in your shoes though, I would go for the biggest tank you can get, and keep multiple smaller fish as opposed to trying to get the largest fish you can in the tank. It will be more enjoyable most likely, as a new hobby you may want to keep adding things, and get bored of a single fish after a while. Something like a pair (or nice solo male) of Sajica cichlids in a 55 or 75 with a bunch of smaller fish would be a fun setup.