What types of fresh water fish would you put in a new tank, if you got one?

I got a new 5 gallon tank for xmas for the office, and it has a pump filter and i got a heater for it (because our office gets kinda chilly in the winter time), and the gravel...... I'm used to fish, so i don't need info on how to take care of them, i just want to know what kinds of freshwater fish you'd put in a new tank if you had the opportunity?