Lots to choose from but splashing tetras (Copiena & Copella sp) are interesting due to the way they breed. They lay their eggs on plant leaves hanging above the water. The male swims below and splashes water onto the eggs to kepe them moist. When the eggs hatch, the babies drop into the water and swim away.
Swordtail characin (Corynopoma riisei) due to the fin filaments and way they breed.
Black phantom & red phantom tetras due to their colour, peaceful nature, and nice fins.
Emperor tetras for their colour and general hardiness (at least they used to be tough, not sure these days).
Congo tetras for their size, colour and movement.
African longfin tetras for their markings, size and peaceful nature.
I had 12 of them shipped to me overnight a few months ago. They came in smaller than expected but are growing great. I have never had them before but they seem a sturdy little fish. These guys are very gregarious and vigorous. After being acclimated they settled right in and ate well that night even after being jostled about on a delivery truck. They haven't stopped being little pigs, they are veracious.