I love Africans, i'd recommend them before anything. I love Lake Malawi personally, much more color and options to choose from.
Mbuna are always good starter africans and still can make one heck of a tank. They are always up to something. Imagine your favorite new world cichlid on PCP.
I'd go Mbuna myself, Haps and Peacocks are more open water swimmers are a 55gal doesn't have the best measurements IMO. Though depending on species of Mbuna, you might get away with 1 or 2 mixed in.
As far as species is concerned for Mbuna...take your pic from the hundreds haha. Really depends on what you can get locally or from breeders/internet sources. Your LFS should have the run of the mill "Mixed Africans". Which is like the humane society with dogs. Nothing really wrong with them but they are most likely mutts or crosses, might find a gem in there sometime, so to a purist, they arn't the most desirable fish. Yellow Labs, all of the "Zebras" are good starters, cobalt blues, so many to choose from.
Advice? Take your time and pick out some species and get the info on them to make the best possible mix of fish. We are all impulse shoppers i know its tough to resist. And try and get most of the fish at once, or at least i would. Kind of helps you in the long run and you can weed out the trouble makers as they come along. At least try and get the same size fish. And if you do decide to go Mbuna, be sure to work in some veggies into their diet.
Lots of rock!