I completely agree on the beauty of the Odessa Barb; I have always wanted some of these, and a few months ago I got a dozen for a fairly cool-water tank whose main focus was a couple of Gymnogeophagus species. The barbs were really just a sort of after-thought, but their colour and vivacity has really brought that tank to life. I know, they don't "belong" with South Americans, but I got over that quickly.
But Odessas aren't really under-rated, because I don't see them often and they tend to be highly admired by people who have them. To me, a fish is under-rated only if it is cheap and dirt-common and taken for granted, but you just know that if it were rare that people would be fighting to get them...either because of beauty, interesting behaviour or some other attribute(s).
Some examples of that would be Cardinal Tetras, Red-tailed Black Sharks, Sailfin Mollies, Angelfish (yes, plain ordinary freshwater Angelfish...), and many other gorgeous fish that we take for granted because they are always available for cheap. Imagine if the Cardinal Tetra (or Neon Tetra) had never been discovered before, and it suddenly hit the scene now. It would be a hot commodity!