Interesting to read through the responses... I like hearing why people like a particular fish, especially ones that I am not that fond of. It shows our diversity in the hobby.
I'm surprised that no one has yet mentioned rainbowfish. There are so many beautiful species of rainbowfish. I have a couple of large adult Bosemani and their colours are amazing. Generally they are active but not agressive.
I also love my flagtail prochilodus with his shiny silver body, large eyes and brightly coloured striped tail and fins. He has a very interesting personality and is quite personable.