There's so many but I'll name some of my favorites. My number one winner is the rainbow trout. I love them. It's a shame that you need a zoo sized aquarium to keep them, but if I could, I definitely would.
As for actual freshwater fish you can keep, discus are second. Bettas are high on the list, but their aggression can make them hard to keep in community tanks. Neon tetras are beautiful and their schooling activity makes them a delight to watch. Fancy guppies are always a favorite, but I find them a little small to catch my attention. There are some really colorful Gouramis out there, but I haven't kept any.