Been keeping cichlids, predators, and etc for many many years now.
Obviously every fish has their own individual temperament.
Cichlid wise, if I were to rank a top 5.
It would be
1. Parachromis dovii - Doesn't need much explanation. Predatory, packed with teeth and size. Even juvie Dovii will slowly climb the ranks and challenge the alpha or next in line, it's only a matter of time before they're at the top and will wreak havoc on the rest of their tankmates. Hate cleaning it's tank, what a mess and kind of scary.
2. Amphilophus labiatus/citrenellus (Midevil, Devil, Midas) complex - Bad to the bone, relentless chasing until whatever they're after succumbs and dies. Very territorial and will fight to death at times. I'd say they can beat Dovii's on average when they're the same size. Only Reason Dovii is higher is because of their equipment.
3. Tilapia Buttikoferi - Butti's are interesting, coming from Africa they present a completely different fighting style to it's American cousins. Ripping off scales, slamming their opponents into things, very persistent and will not stop until they're the top dog of the tank. A very HYPER dominant species I noticed.. Also have seen them dominate dovii's and red devils. I ranked them lower simply because I've had a few butti's that were fairly docile but if you have a dominant one you'll know because all the fish will have missing scales and will be cornered.
4. Nandopsis istlanum - These guys are fierce!! The males of this species fight for sport I swear, I rank them slightly above Grammodes because they will fight down to their last scale. Istlanum is by far the most aggressive Nandopsis species I've ever owned and I've had them all. The couple istlanum I had really put a whooping on umbees, dovii, red devils, grammodes, and much more. I only ever see them lose when there is a substantial size advantage.
5. Nandopsis grammodes - Cold blooded killer. Grammodes are mini dovii for a reason, if death itself was a fish it would be a grammodes. They come off like skittish sissy fish as they hide so much, but then every morning a new fish is dead and torn apart. Grammodes are capable of taking down much larger opponents, they tend to be very skittish don't let that fool you as they will eliminate an entire community very quickly if you let it.
Obviously this is a cichlid list. But the baddest freshwater fish is by far a Rhom Piranha (black piranha) it's not even debatable whatsoever.. There are many videos on the net to prove that, they absolutely shred and destroy anything that dares to challenge it. I'll send some videos in the next couple of posts if anyone is curious, this is not for the soft hearted.... beware.