Haha, thanks to this thread I just spend an hour browsing the internet trying to find out what is and what was the most bad ass animal ever....
(or to say it with TTW words: the GOAT)
I've come to the following conclusions:
current:
land:
African elephant – the sheer weight of up to 7,5 tons would probably be too much for anyone to take. For instance a Grizzly is powerful but with less than a 10th of the weight the elephant would just run over it.
Water:
I would say killer whale or blue whale.
The killer whale owns the white shark so that one is off the table. The blue whale is not a hunter but due to it's enormous size (<200tons) killer whales (<6,5tons) apparently don't attack a fully grown one (much less alone, they usually hunt in groups).
air:
Going by bigger means better I suppose it's the bald eagle
All time:
land:
Hard to tell since all these dinosaurs are extinct and many things about how they really looked and behaved we don't know many exaclty.
My first guess was T-Rex, Triceratops (those two actually fought and it seemed like a somewhat even match), Allosaurus or Spinosaurus.
But then there is the Argentinosaurus who might not have been a hunter but can bring up to 100 tons to the table. Hard to imagine how a < 7 ton T-Rex could handle that much power.
water:
Has to be either Megalodon or Predator X. Both have an estimated weigh of ~50 tons (great white <3,5t). Megalodon apparently has the strongest bite in history ever with up to 180kN with Predator X being clear second with ~150kN (for comparison: great white: 18kN, T-Rex 31kN) but again there is lots of uncertainty.
The blue whale might still be heavier but I think that wouldn't be enough against that bite force, agility etc. of these hunters
Air:
Again there is not much material on hypothetical air fight but apparently the biggest ever was the Quetzalcoatlus with up to 250kg and 12m wingspan so that would be my bet. (for comparison: a bald eagle weighs just a lousy ~6kg)