In a 55 gallon, you have a bunch of options. Here are some:
(1) Mbuna tank (african cichlids from Lake Malawi). Lots of rocks, very colorful, active, and overstocking is purported to reduce aggression.
(2) Peacocks and Haps. More peaceful African cichlids in this group, still get beautiful but check the max. adult size for the haps, some get BIG.
(3) Relatively peaceful American cichlids. Go with some things like rainbow cichlid (herotilapia multispinosa), thorichthys (meeki, the firemouth, is only one kind. Most are beautiful), sajica, spilurum, aequidens (acara. But avoid the "green terror" or gold saum). You could have tetras, rainbows, silver dollars, couple small fancy plecos.
(4) Severum tank, with a couple bichirs (polypterus senegalus is a nice choice, and the albino version is available locally in Tampa at least), small catfish, and maybe some silver dollars.
(5) Aggressive tank - Jack Dempsey and Convict or Green Terror and some target fish. Personally, I don't do aggressives in tanks this small but many report some success.
(6) Dwarf cichlid heaven. Rams, Kribs, Apistograma (like Caucatoides). You could have barbs and tetras, cory cats, and even a couple oddballs as long as you don't get large-growing predators. Be cautious of water: Rams and apistos are not tolerant of poor water typically.
(7) It might make one very cool angelfish tank. Not my thing, but I see why my gf does it.
Hope this helps.