i say poeciliadae livebearers, bristlenose plecos, convicts, any barb or tetra pretty much, yellow labs or smaller sp of mbuna if you want to get into african cichlids
bettas are too slow and expensive to breed for it to be worthwhile imo, hard to get a good pair that won't produce scuffed fry that you can actually sell off
goldfish are just expensive to breed in general
not the fish themselves, just the setups and food/water/time requirements