It's so hard to pick a favorite... They all rock.
Bettas will always be special to me. They're almost not even like fish, they're like their own completely different animal. They just act as though they are higher on the plane of intelligence than any of their tankmates. They astound me.
I love neon tetras and glofish (or zebra danios, for that matter) because I love watching them chase eachother around and play in schools.
So much like little kids playing tag!
I love my fiance's dojo loach and bamboo shrimp too. They are unique and a bit scary, but still terribly fascinating.