what appeals to you
For a 55, I would lean towards medium size fish like Firemouths. Rainbow cichlids might also work. Kribensis. I assume you only want one species in the tank?
There are other considerations, like the water in your area. If you live in an area with hard water, it might be best to keep fish that like hard water, like the ones from the Rift Lakes.
I would love to have a new 55. I could think of lots of uses for one.