The Earth and Sky Watchers of the Silent Realms were absolutely terrifying. Their movements when they were in relentless pursuit was very effective in invoking consternation within me, especially when the cold, ethereal atmosphere and music are factored in.
I found Lanayru to be intimidating as well, albeit in a less stereotypical way - not only for his sheer size, but for his deep, rotund voice. I'd imagine how he'd appear if he were real and how his voice would cause earthquakes. I guess I could say it was much less him, and more so his voice that I found to be intimidating.
I found Demise to be most intimidating. Despite my having considered it to be a somewhat of a joke combat-wise, I did find it to be threatening in appearance and demeanor. Its dark, menacing appearance and size did a great job at instilling a sense of fear in me. My wild imagination might also have to do with it as well. Especially since I'd liken him to Amatsu-Mikaboshi of Japanese mythology, whose description is quite similar. I'd also think about Fi's description of it: (paraphrasing) She's says 'that it appears differently depending on who sees it.' To me, considering the fact that Demise is evil incarnate, how could I not find it intimidating? Imagine facing evil itself.