Are we talking in general, or personal top 10s? Personally...
1) GBRs - gorgeous and fun personalities
2) Loaches (just about any of them) - clowns of the tank, fun patterns
3) Puffers - great personalities, adorable faces
4) Mexican Dwarf Orange Crayfish - funny personalities, cool to watch
5) Angelfish - elegant, personable, doesn't lurk
6) Bettas - smart, variety of personalities, colorful
7) Threadfin Rainbows - cool displays & colorful
8) Oscars - wet puppies
9) Guppies - so many pretty colors and patterns
10) Danios - super active, lots of fin and color/pattern varieties
11) Cardinal Tetra/Neon Tetras - so colorful and hardy once you get past the initial period
Okay, so that's 11... Couldn't decide