Iron Monger, Justin Hammer, Whiplash, Abomination, Frost Giants, Red Skull, the Chitauri and Loki. These are the primary villains from the 1st phase of MCU films, "Iron Man" - "The Avengers", and how many of them have you recognized apart from Loki?
Loki is by far the best villain we've had in the MCU and it's because he has what most of his colleagues do not: charm, sympathetic qualities and character. The audience understands Loki and his jealousy for his brother, Thor. Loki is a joy to watch on screen because of his charisma and well timed sense of humor making him memorable as well. Most of the other MCU villains are far from memorable. When Ronin is on screen in "The Guardians of the Galaxy" conveying his generic evil plan to destroy a big city the audience doesn't care; they want to get back to the heroes of the story which Marvel does a great job of developing. The audience could give a rat's ass about the MCU villains because they are underdeveloped. They don't have any depth to their characters or they haven't been given enough time to develop due to being killed off or lack of screen time leading to them becoming utterly forgettable unlike the heroes. That is the folly of villains in the MCU.
I am going to analyze major Marvel villains from "Iron Man" to "Spider-Man: Homecoming" and explain what went wrong with the characters and how the issue could've been fixed to make a meaningful villain.