I'd say this comes down to the Joker and Darth Vader, but the reason I'd give it to Vader is because as frightening as the Joker is, it's not impossible for a normal human to escape him whereas with Vader you're screwed if you don't have the force yourself. The other characters are more freaky than intimidating.

I would say Darth Vader is the most imposing and intimidating. Pennywise may be a lovecraftian horror that preys on fear but to me it is not really intimidating or imposing because it is a spineless coward beneath all it’s forms.