His size was intimidating
He was a bruiser and when he wrestled with Oley he would destroy people. He follows in the footsteps of those who came before him.
He was a bad bad man, who was willing to pick apart his opponents to get to the top. Not only just that, but he was intimidating because he could hurt you bad in the ring. Not only is the character intimidating by what he is willing to do to you, but he is intimidating by what he will let you do to him. All personal feelings of malice aside, Chris Benoit was a very intimidating man. He was willing to drive his head into your sternum. He is the first man to ever pin Andre "The Giant" in North America.
Then add into it his Legacy as well as what he was able to do when he was young and healthy, you have one intimidating man. Who cares if he barely made it through Wrestle Mania 3? His size and physique alone are intimidating, just ask any of his MMA combatants, but also his in-ring abilities, his next big thing gimmick, and his lasting legacy of what he has done in the WWE have made him the most intimidating man in WWE history.
There have been times where he has not been intimidating, but the times that he was, no man from his era, except for number one, was more intimidating. No man has come close to his height and size within the ring. He scored the highest total amount of points for intimidation combined.
Though many disagree that the Samoan Spike is an intimidating move, let someone drive a heavily taped thumb into your throat and come back and talk to me.
For a man who carried a spike to the ring with the sole purpose of it going into his opponents skull, many people do not remember him as intimidating. Go watch some matches of the original Sheik and see why he is so intimidating. His "lack of language" was a great intimidation factor as well as his ruthlessness.