I have always liked "fish shaped" fish, so although keeping very many types of fish over 40 years, I always end up back with barbs, rasboras and the other similar Cyprinids and, to a lesser extent, Characins. I don't like aggressive fish, and appreciate elegence rather then flashy colours.
If I had to pick a species, it would be Barbus gelius. That little fish was the topic of my breeding programs for about 5 years, (set up to prove an idiotic reclassification wrong). No matter how familiar they became, (I had 10+ tanks filled with them), they never lost the attraction I had for them when I first saw them.