That would be the saltwater crocodile. Bite force is partly about raw size, but it also has a lot to do with the shape of the jaw and the position of the muscles. Bite force hasn’t been measured directly for every known animal, but those we have measured allow us to estimate the bite force of the others, simply by measuring their skulls.
Humans bite with a force of about 890 Newtons (200 pounds force), while lions and tigers bite five times harder at 4,450 Newtons (1,000 pounds force). At the top end, saltwater crocodiles’ bite is almost four times harder again at 16,460 Newtons (3,700 pounds force).