Dr.Doom- second least intimidating. He's actually one of the most reasonable that are listed. Sure you don't want to make him angry, but if you have a valid enough point of view, then he'll let you go. He also respects people with high intelligence or very good morals.
Darth Vader- Is he intimidating? Hell yeah! Is he the most intimidating one here? Nope! Frankly, vader is the second least intelligent person here and is very vulnerable to manipulation. If I met him, I could easily pull his strings while simultaneously pissing him off.
Loki- definitely the most intimidating. He hates mortals and sees them all as pigs for his slaughterhouse. He's a master manipulator with the power to back up his genius. He's also been known to have a bad temper.
Bizarro- the only person here that you can consistently insult and get away with it. Super easy to manipulate and roughly as strong as superman to boot.
Yeah...reason why I chose Bizarro is because you literally have no idea what would set him off. Doom/Vade/Loki - they're pretty easy to predict. Don't get in their way, and don't be a threat, and you'll be fine.
Bizarro? If he takes a liking to you (for whatever warped reason) he could end up force feeding you literal sh!t (because it makes sense) or hugging you so hard you explode, or just straight up throwing you into the sun.