This isn’t even a question! Bettas!
I've kept so many species of freshwater and saltwater fish with great success but bettas i just can't keep alive. Tried about 5 times they usually die within a few hours of getting them home. Even bought them from different sources with no better luck. Maybe something in my water they don't like? who knows.
Dwarf Cichlids, especially from South America
Probably bettas, hatchetfishes or kuhli loaches for freshwater. For brackish, mudskippers. For saltwater, probably convictfish, or maybe warty frogfish. Do you mean no BETTA luck?
Depends on the style, I really like tuna, then salmon. But the other way I prefer snapper or grouper, maybe striped bass. /wink