My vote goes to the zebra danio. It is a gorgeous fish (when they're in breeding mode, their blue becomes purple and the yellow turns shiny gold...The long finned forms are spectacular in that phase) that can tolerate a wide range of water conditions and temperatures, is very cheap and hardy and easy to breed. I think they have a leg-up on female bettas both price-wise and on hardiness. Theyre a good pet fish too heh, they fearlessly nip at your fingers when you reach in the tank to rearrange stuff and eat from your hands with gusto. 
