Why has it been so hard for other MCU villains to match Hiddleston's performance? Pretty bummed to hear that Mads wasn't used to full potential in Doctor Strange.
Most of the time, the way the story is set up doesn’t have room for it. There isn’t space for a Tony Stark redemption arc with how much there is to untangle there and for Obadiah Stane to be a nuanced antagonist. Captain America’s fighting a super-Nazi with a skull face, you aren’t going to get much sympathy for him. A villain who’s anything but a one-note moron would be out of place in Guardians of the Galaxy. Iron Man 2 couldn’t make the potentially interesting Whiplash work because Iron Man 2 sucked real bad.
While I’d say there have been a few other notable MCU antagonists (Alexander Pierce, Ultron, and especially Zemo all worked, and I’d even say Red Skull was underrated as a solid ham bad guy), obviously Loki is the best of the lot, and a lot of that does simply come down to Hiddleston acting the hell out of it. If you go back and look, a lot of his lines as Loki are actually total schlock that he elevates to greatness through sheer force of will. For instance:
Thor: Loki, this is madness!
Loki: Is it madness? Is it? IS IT? I don’t know what happened on Earth to make you so soft! Don’t tell me it was that woman?… Oh, it was. Well maybe, when we’re done here, I’ll pay her a visit myself!
That’s the B-ist of B-villain dialogue - straight up “I’ll get you my pretty, and your little dog too!” material - that thanks to Hiddleston is absolutely fantastic, turned to Loki being unable to comprehend or accept that his shitty big brother has changed for the better on the eve of revenge, and realizing what he can say to provoke Thor into being the brute he knows. He even managed to make chunks of Dark World almost not feel like not being stabbed, and it’s definitely fair that a lot of people consider him the acting MVP of the MCU.
Just the way his character is built demands a greater degree of complexity too though. His drive is that he wants to be loved - the most sympathetic motivation of all - and his hatred of Thor in that regard is justified, because the whole first act of the movie is about what a complete asshole Thor is. By necessity he’s a more understandable and nuanced character than you get with your Abominations and Ronans, and what Hiddleston gave to him - along with a position as the villain of Avengers to ham it up for all he’s worth and secure him as a major pop culture figure - sealed the deal.