Pound-for-pound, what's the meanest, nastiest, orneriest, toughest fish you're aware of?
Personally, I'd vote for the more aggressive bettas I've encountered. Some bettas don't care about much, but I've had a couple that, if they weren't so small and basically stuck in wedding dresses, would be man-eaters. I've had a few bettas that did their level best to rip my arm apart. Most other bully fish (sunfish, damsels, etc) turn and run when they're faced with something that actually fights back, but I used to have a betta that would fight my entire arm, its reflection, the 'eye' of my camera-phone, and even the net. Demon betta, that thing.
Personally, I'd vote for the more aggressive bettas I've encountered. Some bettas don't care about much, but I've had a couple that, if they weren't so small and basically stuck in wedding dresses, would be man-eaters. I've had a few bettas that did their level best to rip my arm apart. Most other bully fish (sunfish, damsels, etc) turn and run when they're faced with something that actually fights back, but I used to have a betta that would fight my entire arm, its reflection, the 'eye' of my camera-phone, and even the net. Demon betta, that thing.