Personally, Sully! In being a monster, who can beat HUGE, GIANT TEETH, LOUD ROAR and CLAWS AND HORNS!?! Randall is scary, but 更多 in life with a personality than being a monster. If I met Sully in my bedroom, I'd be scared to death and faint, if I met Randall, I'd probably hit him over the head with a 网球 racket and say "OMG, BABY CROCODILE! I gotta call Wires!" Randall's just...a bad guy. Sully's scary.
Randall is 更多 scary but Sulley is sweet and good monster but Randall is scary because Boo does not like him. She was so scared about him taking her away from Sulley.
No one saw how Randall scares children so how would I know? We know he's threatening to monsters in general, but he could be scarier if we saw a scene of such.