The most popular supervillain in the Marvel movies at the moment.
A sexy mofo with a tragic backstory, Loki was (at least in the Marvel cinematic universe, I do not vouch for comics) left to die by his birth father Laufey, the king of the Frost Giants of Jotunheim, because he was the runt. Odin took him in both out of compassion and because he thought adopting a Frost Giant would be useful to him in improving relations between their two kinds. Loki grew up being treated like something of an outcast by his adopted dad, who never told Loki that he was adopted and always favored Loki's brother Thor. This made Loki extremely jealous and desperate for his father's approval, which led to his becoming a villain. That and the fact he's the God of mischief and lies... so with that as your official job title, you probably aren't going to be the world's (realm's?) nicest person.
So me and other obsessive fangirls like myself are in love with him because he's beautiful and tragic and possibly even a good person under all that. And he's played by Tom Hiddleston who is God in a human form. You can't top that.
Loki is more popular than Thor because Thor is an arrogant-ish little goody two shoes and Loki is a MAN with fabulous dark hair and a sexy smile. He's also a bad boy, which makes him twice as hot. And British... so three times as hot.
Girl 1: TUMBLRRRRRRRRRRR.
Girl 2: Has she been Loki'd?
Girl 3: Of course she has. Loki is beautiful and perfect.
Girl 1: *cries* I DON'T HAVE A PROBLEM OKAY?