Ooh, ooh I love cichlids, I am in the process of getting rid of my 'smaller' tanks as I have just got a new 130G, in the picture gallery if you wish to see. I have a combination that, a very disorganised combination, comprises of South and Central Americans and a couple of Africans, Malawi I think.
The americans, at least the ones I have, are not nearly as colourful as the Africans, specifically the Malawis but then the Tananikans are also very beautiful.
Try these sites for more info on cichlids:
Personally choosing a cichlid would be down to each individual, I could suggest some cichlids but then I would be biased to the ones I recommend as they would be what I would consider my favourites, and they might not be yours?
There is also the dwarf cichlids to consider, but a 55 you could look at getting a couple of fairly niced sized fish once they are older.
Good luck, hope you enjoy your ciclids as much as I do
PS I forgot, I was too late when I discovered these beauties but maybe you will love them as well?:
This is one of my beauties: