Loki, "Thor: The Dark World" - What's so unsettling is how calm he is. How reserved and determined. You know he's serious and you know you never want to piss someone like Loki off. There is a reason the silent rage is the most feared.
Now all I can think of is an AU where Odin kept Loki as a literal relic locked up with the other stolen Jotun things in the vault. One day, Thor and the Avengers finds a half-crazed Loki in there when researching the Tesseract.