Anything that can bite down hard enough to pulverize Triceratops bones or crack an ankylosaur shell is by definition scary as fuck. T. rex is definitely a good pick.

Personally i'm more scared of large azhdarchids like Quetzalcoatlus, tho. Just look at that beak, it's a giraffe sized marabou. ever seen marabou storks go to town on a bunch of offal? Absolutely foul, ghoulish fuckers. The idea that one of them could be large enough to just swoop down and impale a human with that murder weapon of a beak is absolutely the fuel of nightmares.