A green terror would do well in a 55 by itself. They have impressive coloring, especially the males, with a thick bright orange-red outlining on the anal and dorsal fins.
Plus you could keep a common plec, or a synodontus or something as a bottom feeder.
Have any of you ever seen an oscar with this paterning? just the coper-orange background (common) then lots of black dots in straight lines left to right all over its body. I saw one like that at wal-mart and thought it looked intresting and was 5". we can't forget its wal-mart. it had a pach of fungus on its head. then some idiot with a 20gal buys it with another 5" inch oscar that I would say looked great (I rarely say that about wal-mart) if it didn't have ich