Looking at a lot of these animal fighting videos--the ones of wild animals supposedly fighting each other--I get the sickening feeling that a lot of them were deliberately set up and staged.
For pure pound for pound ferocity and tenacity when fighting, I don't know if there's anything much in the animal kingdom that can top a well-bred fighting dog like a Pit Bull or a Tosa. I got my current Tosa b#tch because when she was with her original owners, the neighbors' 100-pound Akita b#tch came on her property. Both females were in heat at the time. The upshot of it was that Tessa the Tosa killed and ate the Akita, and the owners couldn't handle it and gave her back to her breeder, a close friend of mine, who gave her to me after I had to put down her uncle for health reasons. Yet she is the sweetest, gentlest dog you could hope to find. If she didn't have the scars to prove it, I could scarcely believe she was a killer and a cannibal. She is wonderful with little kids. She will often lie down and let them roll around on her chest and tummy. I bet their parents would freak if they knew her history.
Her father was a pretty tough customer as well. One day the neighbor's Rottweiler, a large, well-bred Rott, made his way over and began fence fighting Bosu. Bosu smashed his way out of the sturdy kennel run made of heavy gauge wire and crushed the Rottweiler's skull. As a make-good Rick later gave his neighbor a young Tosa, so now the neighbor has a "real" dog.
Tessa's mother had to be put down after an accidental kennel fight with Tessa's aunt (her father's sister). The "winning" b#tch required $1,100 in veterinary bills (although Rick may have been ripped off by the vet). That must have been one hell of a fight!
I have heard of an incident in Japan in which a Tosa was pitted against a leopard and prevailed, although I find it hard to believe. If true, I suspect the leopard was a good deal smaller than the dog.