That really depends on what kind of fish you like. I have guppies and platies together right now, but will be getting rid of my female guppies once the weather warms up and I can ship them (a member of the forum wants them).
You could do a nice centerpiece fish like a dwarf flame gourami or neon blue gourami, and a clean-up crew for the bottom level of about 4 cories (the smaller varieties like peppereds and albinos, not the ginormo emeralds).