A lot of really nice fish are mentioned in this thread.
To choose my favorite fish, I need to pick one that I've owned as there's more to a fish that just looks.
I know that Clown Loaches are quite common, but they'll always be in my top favorites. I've seen a couple that are over 18" and they're always so playfull when they are in a group.
My favorite fish though is the Flagtail Prochilodus. Here's mine with some clown loaches.
If you want to see them in action, check out this video:
http://www.youtube.com/watch?v=0W64BrTu2bc