I like Rift Valley cichlids and the bright "neon" colors of the small rasboras and tetras, as far as fish that are classically listed in this type of thread.
There are a lot of fish that get left out of these threads simply because they're beauty is subtle and only comes out in the best of conditions. The cupid cichlid (Biotodoma cupido) is one of these; I've never seen a picture that does justice to the beauty of these fish in an aquarium. The rainbow iridiscence on the sides is hard to catch with a camera and can compete with any fish in the world. A lot of the wild-type livebearers fit here; fish such as the montezuma swordtail (Xiphophoros montezumae), Ameca splendens, and the merry widow (Phallichthys amates). A sparkling school of diamond tetras is hard to beat. Almost every fish I've ever kept has had qualities that could put it up there in the competition for "most beautiful".
Well, I normally don't care for hybrids, but the OB Peacock is a stunning and fully functional exception. I believe it's a cross between a mbuna and peacock. Metallic blue and orange with attitude!
By far my most favorite fresh water fish is my oscars. Nothing cuter than watching those fattys beg for my attention....usually just for extra food lol.