There's a lot people like this, mostly Akatsuki members.I am not talking about who is the strongest, but someone who is extremely skilled and dangerous to be around. "Oh, people run for your life" just an example.
You could be afraid of Orochimaru experiments
Kakuzu ripping your heart out
Hidan's ritual
Kisame cutting you with unwrapped samehada
But all of them pale in comparison with bloodlusted Itachi
endless tortures inside tsukuyomi world > everything else.