If you have a good size tank, rosy barbs are great fish. The males are bright red with black fins. The females have a gold spot under the black spotat the base of their tail that almost glows when things are good in the tank. Their active without being spastic like danios.
Rosy Red Feeders are wonderful schoolers and Fathead Minnows are just as awesome. Also most barbs are underestimated, they're colorful and marvelous personalities make them a great addition to almost any tank (if you have the room).
OK since no one mentioned it and I have one, the Gobi Dragon. For its length and size has an amazing ability to hide. I have looked many times under the tank and cabinet looking for mine thinking it had taken a walk which they will if they don't like the water. They have been known to live outside the tank for days.
I'd have to say diamond tetras (seems to be a common thought here at least!) and goldfish. A lot of people get scared off of goldfish because they're "too common" or they didn't know how to care for them and killed them. I loved my fantail but didn't get his pond built to put him in and had to sell him back to the LFS (darn you hubby! lol).
Oh, and snails. Everyone seems to think they are evil, but I just love 'em.
danios i have both leopard an zebra an watchin them swim bout an chasing each other so funny at times an they always come to the tank an say hi soo yea under rated but my fav is gotta be the leopards