If you are going with a species only tank your choices are wide open. A 55 gal gives quite a bit of room for a large group of mbuna.
You could go with a large group of Pseudotropheus demansoni. They are fairly aggressive to their own kind but with a large group like one you could put in a 55 the colony should get along just fine. You could also add a colony of Yellow labs as a color contrast. These 2 species look great in a tank together and get along famously.