#25 - Hopper
"Hopper is a lean, mean eating machine who can squash an ant's head with his foot if the food's not on the offering stone. Hopper's tough exterior conceals an even tougher interior and a superior intellect." according to the official Pixar movie site. He knows the ants are small in comparison to the grasshoppers, but he knows they out number the grasshoppers hundreds to one. I think the clip below is Hopper in his true form.
While Hopper may not have underestimated the ants' strength in numbers, he did underestimate Flik's intelligence, and that was a BIG mistake. Hopper's false sense of superiority got him caught in the end.