Congrats on starting a rift ake tank
The first question is do you want to do a malawi tank, or a tang tank? You could also do fish from lake victoria but they can be hard to find and can be a bit scrappy.
For a malawi tank your options are:
Peacock tank:
Pros: Gorgeous coloration, lots of variation.
Cons: they can be a little hard to find. I also find it difficult to find species that don't look similar. Females are very drab but you could do an all male tank.
A hap tank
Pros: Yet again lots of color. I honestly don't know a whole lot about haps
Cons: Many get too large for a 55 gallon so you will have to pick carefully. You also can't have as many because they are an open water species. Females are pretty drab but you can do an all male tank.
You can also mix haps and peacocks for a bit more variation.
Mbuna Tank:
Pros: Lots of color and activity, easy to find. You can have more fish in your tank since they need to be over stocked. Most females are just as colorful as the males.
Cons: Aggression, these guys can be really nasty if you don't get the right combination. Most of the fish have a similar body shape so there isn't much variation.
They also don't mix well with Peacocks and Haps
You could also do a Tang tank.
Pros: Lots of variations. It can be set up more like a community tank. They show a lot of interesting behaviors.
Cons: They are not as colorful as most malawis(for the most part). You also need to be careful that you get the right combination to avoid problems.
I will admit that I'm very partial to mbuna

I don't know a whole lot about tangs but I know many people around here keep them.