I think it’s a bit too much of a stretch to call a bacterium an animal, being that it’s a bacterium.
I think the gorilla is the smartest animal we’ve put forward - he’d get out of the way in a high up corner somewhere and come back down once all the fighting’s over.
Assuming the animals fight with each other on an individual basis in some sort of tournament, the African elephant has the definite advantage in size and utility. He may have to ram his tusks into the blue whale a lot of times to kill him, but the whale won’t put up an effective defense. Now a small venomous snake might take him out in the early rounds, but if the matches are properly seeded, small venomous creatures won’t make it very far through the tournament, and most are probably out of venom in the later rounds.
But if all the animals are tossed into a Battle Royale, then the winner could be any one of them. A little animal might hide and survive while the others killed each other. Several large cats might team up on the elephant and eliminate him early. All of the animals might ignore the skunk as long there was other competition, and when the alligator and the python end up killing each other, the skunk walks out untouched.
If there is water, then the only winner is a Killer Whale, everything else is dinner.
How exactly does the killer whale get the animals that don’t go near the water? Or stop the horned ones from goring it if it leaves the water?
Anyway, there should be seperate aquatic divisions. The orca won’t be the winner there. They’re midgets in the world of whales, but it’s likely to be some variety of whale.
An aerial division would be a good idea too. Birds, bats, and bugs can battle that one out.
Sweet Jeebus on a cracker, how is this not already a video game? 
Another vote for Honey Badger
There is a small wallow for the wallowing animals. Certainly not enough water for a killer whale as I mentioned in the OP.
So, I guess there needs to be different seeds, so how about by species.
Like, all birds face off, then all cats, all canines, all primates, etc. choose the winner of each and then they face off.
Whats the deadliest snake out there? I vote for it.
I don’t know if facing off animals of the same species against each other would work – they might mate rather than fight. Same goes for genus to a lesser degree.
Also, a lot of animals simply don’t fight to the death. Put a chicadee in with a tit, and they would probably both die of starvation or old age before one killed the other.
It might be best to have a carnivore that eats something bigger than worms and bugs in every match, just to keep things moving along.
I’m not going to link it, but there is a YouTube video of an old black and white movie squaring off a bull and a lion. The lion didn’t stand a chance, the bull was just too powerful.
I’m going with a Kodiak bear or a hippo
Is there only one of every creature? Like, one elephant, one tiger, etc.? Because if you put a team of chimps in there then I’m betting on the chimps. They’re intelligent, crazy aggressive, and can work together.
If it’s just one chimp, then I’ll have to think about it.
Just one of each, and I agree Muffin there will have to be something carnivorous in every round just to move things along, otherwise the lagomorph round is going to last a LOOOOOOOOOOOOOOOOOOOONG time.
Bored!
I think I’ll go with the hippo then, if only because of a thick hide and pretty strong and powerful.
A hippo could blow away much of the competition.