We would love to help, but there are so many different kinds of fish to put in a 55, it would take all week and 16 pages to list everything.
that question is just too broad.
I aggree that you should take a trip to the fish store, petco, petsmart, etc. Look at all the fish they have.
(keep in mind they are all babies and some will grow huge while others will stay small)
Usually they list under the tank what type of fish and how large or aggressive they become. (these are just guidlines, many of these listed facts they have are not accurate, but they give you an idea)
You'll need to decide if you want small community fish or 1 or 2 larger growing fish.
If you want community fish, you have a mile long list of possibilities and combinations.
If you want a lone fish, Oscars, Jack Dempseys, Green Terrors, are some possibilities.
Try to research it a little more first. Then what we can do is help you NARROW your choices down to something appropriate.