1 He is the Marvel's Joker
Loki from the Marvel Universe is the counter-part of the Joker from DC Comics. The Joker is one of the most likeable villains of all time, according to several lists. (See Why do some people like comic books?)
2 He's funny
The representation of Loki in the comics makes him a fun villain with a humorous side.
3 He's based on Norse mythology
One of the reasons why Loki is so popular is because he's based on a character with the same name from Norse mythology.
4 He is a surviving villain
Through the years, several villains from the Marvel Universe have died and others replaced them. However, Loki is an exception. He continues to haunt the Avengers and Thor all the time.
5 He was well presented
Loki was presented by Tom Hiddleston who did a great job making the villain ruthless and likeable at the same time.
6 He uses illusions
One of the powers of Loki is using illusions to trick people. He is originally a trickster god who manages to fool even Thor and Odin.
7 He had a tragic backstory
Loki is not the biological brother of Thor. He has a far darker story that ended with Odin adopting him.
8 He is too powerful
Loki is a very powerful character who manages to stop the Avengers all together, even in the movie version of the Avengers, Loki manages to trick them all.
9 He supposedly kills Odin
Odin is supposed to be eaten by Fenrir in the myth, the Son of Loki. In the movie, Odin is killed by Loki who replaces him and takes the throne from Thor without his knowledge.
10 Girls like Tom Hiddleston
Girls love Tom Hiddleston who is a great actor and a good looking one with a bad-boy personality. It is one of the reasons why he has gained so much popularity along with his characters. (See How To Dress Like A Bad Boy?)