Removing Humans and ignoring things pathogens and viruses, the more I think about it the more unsure I become.
Logically it would be something large like a Hippopotamus or Elephant, but if they get ambushed from above by a Siberian Tiger or bitten by a King Cobra they are done. A saltwater Crocodile will shake off most things but would it get squeezed to death by an Anaconda? A Polar bear could kill a Lion but not a whole pride. Don't worry about "It wouldn't do that" or "They'd never meet", make them all rabid and hungry if you want.
What does the wider world think would be the true Apex animal if everything got into a fight for survival using their biological tool kits?
Logically it would be something large like a Hippopotamus or Elephant, but if they get ambushed from above by a Siberian Tiger or bitten by a King Cobra they are done. A saltwater Crocodile will shake off most things but would it get squeezed to death by an Anaconda? A Polar bear could kill a Lion but not a whole pride. Don't worry about "It wouldn't do that" or "They'd never meet", make them all rabid and hungry if you want.
What does the wider world think would be the true Apex animal if everything got into a fight for survival using their biological tool kits?