....In terms of personality, hardiness, and appearance?
I am partial to Celebes Rainbows myself. I'm actually putting them into my 40 gallon once that is cycled. Everything I have read on them has said how peaceful they are. I also saw some in a not so local fish store and they are very pretty.
They are often overlooked because of the soft colors and the stores don't display them correctly. But once established, and in the right setting, their colors are beautiful. I love the lemony/ish colors on them with the blue/ green line going horizontally down the body is beautiful.