"I am Loki of Asgard, and I am burdened with glorious purpose." Ever since his appearance in Thor in 2011, general audiences greatly received Loki, played by Tom Hiddleston. In fact, out of all of the villains in the Marvel Cinematic Universe, Loki is the only one who is considered a standout. Moreover, fans have been demanding a Loki standalone movie. But here's the thing, we already got a Loki movie in the form of The Avengers in 2012. While the God of Mischief was the superhero movie's main villain, he was also its main protagonist, if you watch the movie from a certain point-of-view.
While five films (Iron Man, The Incredible Hulk, Iron Man 2, Thor, and Captain America) led up to The Avengers, you have to consider that one of the movies (Thor) was in equal parts the God of Thunder and his adoptive brother the God of Mischief, Loki. The film is about two brothers vying for the throne of Asgard, as one fights towards the side of good, while the other draws closer to the evil side. Loki is not a true villain, but rather a tragic and fallen hero. It's one of the reasons why Loki is so clearly defined and why he's considered the best villains in the Marvel Cinematic Universe; we can relate to him as a flawed individual.
Almost all of the other villains in the Marvel Cinematic Universe are forgettable and not relatable because they're not seen as tragic figures, but rather faceless evil characters with very little to no personality to them. Can you telling me anything about Obadiah Stane from Iron Man, Dark Elf Malekith from [Thor: The Dark World](movie:206462), Maj. Emil Blonsky from The Incredible Hulk, or Ronan the Accuser from Guardians of the Galaxy? Most of the villains are disposable and faceless. Then you take a villain like Loki, who is cold and calculated, but shows genuine emotion and pain. Loki is a very attractive character because we can sympathize with his pain and loss. He was passed over for the throne because of who he is (through no fault of his own) and all he wants is just to rule a world. Granted we cannot empathize with Loki's endgame, but we can sympathize (or understand) his resolve.
Now look at The Avengers. If you follow a very broad definition of who a movie's lead character is, then Loki is clearly the Marvel movie's protagonist. Loosely, when a movie begins and ends with the same character, then that's almost certain that's your man (or woman). Now, this rule doesn't pertain to all movies, but it's a good guideline to figure out leading and supporting roles. The Avengers begins with Loki receiving a scepter and invading S.H.I.E.L.D. Loki also drives and pushes all of the movie's forward momentum and story, as he becomes the reason why The Avengers assemble.
While Iron Man, Thor, Captain America, and Black Widow and Hawkeye are really the main attractions for The Avengers, they really don't serve any narrative or thematic arc (other than Tony Stark's sacrifice at the end of the film). The superheroes really serve as window dressing, as Loki is the only character that has a full dramatic arc, albeit a very tragic one. While it's true that the story arc focuses on The Avengers coming together to work as a team, it's not nearly as interesting to me as Loki's arc to claim his "birth right" as a ruler of a world. It's really a sad and unfortunate end and realization of a wonderful and complex character. At the beginning of the film, Loki wants to take over the world, but by its end his journey proves fruitless, as Thor brings him back to Asgard as a prisoner.
Throughout the entire running time of The Avengers, Loki shines through and through when he's on the screen. Although he is a villain and is evil, he's still sympathetic and likable, which is why we all want to watch a proper standalone Loki movie. There's a reason why no other villain in the Marvel Cinematic Universe is just as likable and pleasing to an audience, which is why The Avengers is considered the best Marvel movie yet. Tom Hiddleston goes a long way in making Loki a fan-favorite, but a lot of good faith comes from the villain's tragic ambitions and his drive to do better than his champion brother Thor and king father Odin. Loki is flawed, but that's why we like him. Just remember, "You were made to be ruled."