I was going to say Ghirahim, but then I saw the Guardian option. Holy crap, I forgot about those guys. They're by far the most intimidating beings in the game because you can't do anything to them. Your only hope to escape them is to find a Sacred Tear to get 90 more seconds of not being chased. But then you have to avoid the Watchers! Those guardians are just plain scary. (Yet awesome.)
For this, I would have to say the Silent Realm Guardians. They Scared the SHEIKIAH outta me!!! I'd have to say, out of all, the Silent Realm(s) was one of the most terrifying points of the game in all. Especially when the music, colors, and ambiance changed; the Guardians awoken and started CHASING you until you found the next tear, it seemed like you were running a race against your life. And if you didn't go fast enough, or your energy gauge was fully depleted, let's just say, "The Hero Shalt Not Return. Until They Start Over, For The 14th Time, And Counting."
Awww...no love for Stalmaster? I almost picked it because I always found myself keeping as much distance as possible during that battle, expecially during Hero Mode.
I picked Silent Realm Guardians though because I didn't even want to start the silent realms until I knew the best possible path for collecting tears. I did NOT want to be on the business end of a Guardian's weapon.
I haven't fought demise yet so I can't say anything about him. I said Ghirahim as he was hard to beat and pretty freaky at times, especially when he was doing weird things with his tung or going on a rampage about things not going his way.
But now that I think about it I would defiantly say the Silent Realm Guardians because those things just stalk you till they finally hunt u down and their music is pretty freaky.
I'd say Ghirahim is the most intimidating character, simply because he is so open about his attacks, and very up-front about his intentions. When someone has that much confidence, you know there is something to be afraid of.