Being a purist, or as much of one as possible i don't like other fish with cichlids, just cichlids
However there are some exceptions like the more community type cichlids (Angels, rams, apistos, etc.)
For a 55gal however i would reccommend Africans. I'm a big Malawi guy, Orion is a big Tang guy so i'm sure we will both push for those lakes
Malawi holds most of the colors and activity IMO. A tank full of Mbuna or Haps pleases my eye. In my experiences, fish from Malawi hardly every stop moving. There is always minor scuffles over territory, flashing, and just lots of activity. The fish from Lake tang hold a lot of personality but lack the bright colors, they are however no less fun to keep.
Since i love Malawi so much i would recommend Haps or Mbuna from Lake Malawi. Usually a lot of african beginners start with the "Mixed Africans" at their LFS. Mixed africans are for the most part Mbuna.
I personally like to stock 12-15 Mbuna in a 55gal tank, and possibly 1-2 Synodontis catfish.
Stocking Haps would be a little different because they can get a little bigger than most of the Mbuna. And all mentioned above would really prefer a more rocky habitat without the plants.
If you want plants then you should look at the South American, Central American, or West African Cichlids. Kribs, Rams, Apistos, Angels, Discus, Convicts, the list goes on and on. Some of these can be mixed into a community type atmosphere like the Angels, Apistos, Rams, and Kribs.
Sorry i wrote kinda of a long response, and threw out a lot of names and such. I would suggest you take a look at our Cichlid profiles and see if something grabs your eye.
http://www.myfishtank.net/reviews/showcat.php/cat/5
I'd also you browse some of the articles here, maybe it can give you some ideas or spur some intrest.
http://www.cichlid-forum.com/articles/