So far I haven't found a nano fish that I really find interesting. If I was to pick one it would be embers in a planted tank, which I have now, because they have a beautiful iridescent orange color which makes them stand out against a green plant background. They look quite nice when they school.
To get back to my first statement, the smaller fish can look impressive in the well laid out tank but after a while they are just part of the scenery they swim around but don't really interact. Other larger fish, such as pearl gourami, angels, rams, discus, some of the african cichlids etc, interact more with their owners, and they have unique behaviors that make watching them more interesting.
If I was limited to a nano tank, I would go with a female betta, fewer fin issues, often more robust and healthier than their male counter parts who are too high strung, and they are very interactive with their owners. I have a couple and they come out to meet me whenever I go to the tank. They are one of the fish I have to move out of the way when I vacuum the sand or clean the glass.
Obviously a very biased point of view.