The MCU ultimately did this to Iron Man, Captain America, Thor, Black Widow and Hawkeye. While the characters were always prominent in the comics and had appeared in toylines and cartoons, compared to Spider-Man, the Hulk, X-Men and Fantastic Four they were very much considered B-list. Then Iron Man came out and made Tony Stark as synonymous with Marvel as Spidey and Thor and Captain America: The First Avenger did the same with its titular heroes and by the time of The Avengers (2012) they along with Black Widow and Hawkeye had catapulted into the mainstream and became the Marvel superhero team. Before then, the X-Men (and to lesser extent the Fantastic Four) had always held that title among readers and viewers. Thanks to the films, Iron Man, Cap, Thor, Black Widow and Hawkeye's comics got more recognition and sales than they’ve had in decades.
Captain America also became one within the MCU after Captain America: The Winter Soldier became a hit among critics and fans: For Phase 1 Robert Downey Jr. as Tony Stark/Iron Man was clearly the face of the MCU with the marketing and the story of The Avengers mainly focusing on Iron Man. This changed with Avengers: Age of Ultron where Cap was as much in the focus of posters and trailers as Tony. Cap and Tony are mostly seen as equally important flagships to the MCU now, highlighted in Avengers: Endgame.
Before 2015, Wanda Maximoff / Scarlet Witch had become infamously known for House of M, where she wiped out 99% of mutantkind in a fit of grief-induced madness. People really warmed up to her when she made her film debut in Avengers: Age of Ultron, with her eventually going on to get her own Disney Plus miniseries.
Agent Phil Coulson is the very definition of this. In the first Iron Man, he had a minor role as a S.H.I.E.L.D. agent. Insanely positive fan reaction to his character got his role somewhat expanded in Iron Man 2, and in Thor and The Avengers, he's one of the central characters. Even his own death couldn't stop him, as he just was resurrected to lead the TV series Agents of S.H.I.E.L.D.. The guy headlines his own short films, has his own comic book series, and from the looks of it, is getting bigger and bigger. It's worth noting that he's such a fan favorite that he's crossed universes; Clark Gregg had a few minor spots as an FBI agent on The West Wing and some fan fiction has run with the idea that the characters are actually the same person, and that Phil Coulson was merely undercover in the FBI.
Loki was surprisingly popular after the release of Thor, and by the time The Avengers came around, Loki became almost as merchandise-able as the heroes themselves. Suddenly people wanted more of Loki and actor Tom Hiddleston to the point where Target even released their own Thor: The Dark World Blu-ray packaging that featured only Loki on the cover. In a poll published by USA Today on March 28, 2018, Loki was voted as the most popular MCU character. In November 2018, Disney's CEO confirmed that Loki's set to star in his own show for Disney+ for a 6-8 episode limited series with Tom Hiddleston returning to the role, and in July 2021 was as of this writing the only one of Marvel's Disney+ limited series to get a second season.
The Guardians of the Galaxy, for decades, was a D-list comic book team from Marvel, known only to hardcore comic book fans and trivia buffs. Then, they had a movie, the popularity of which led to a spike in sales of the comic book, a TV show, an explosion of merchandise, a sequel, playable character status in various video games like Disney Infinity and Marvel vs. Capcom: Infinite, a successful standalone video game, a Telltale release and even two pinball machinesnote A digital one form Zen Studios and a physical one from Stern Now, show a picture of Star-Lord and his friends to people, and you'd hardly find anyone who won't recognize them.
Scott Lang, the second Ant-Man, benefited greatly from the movies. In the comics, he was always a rather minor character, only created to fill the role of Ant-Man after Hank Pym abandoned it. Throughout the '80s and '90s, Scott was a supporting character for Iron Man (see above) and the Fantastic Four, and in the '00s he became an Avenger only to get killed off as C-List Fodder not long after (he got better). However, since the MCU decided to make Scott the Ant-Man of the series, he's gotten far greater exposure and popularity than he ever could get before, and has since become the face of the Ant-Man legacy in general, with the comics and surrounding media following suit and thus he became much more prominent than he ever intended to be.
Nick Fury in the comics was originally an old white man from WWII and appeared as such in most media outside of the comic books. However, when Marvel introduced Fury into their Ultimate Universe they reinvented him as an African-American man, using the likehood of actor Samuel L. Jackson for the optics. That lead to Jackson playing the character in the MCU, making his version of the character soon the definitive version of the character: Not only did Marvel introduce a new Nick Fury into their mainstream comics, nearly all appearances of Fury in other media like Western Animation and Video Games use Jackson's likehood for the character. He's even slated to finally take the center stage in the Disney+ series Secret Invasion.
In the comics, Doctor Strange's manservant Wong was a minor character who was entirely subservient to the Master of the Mystic Arts with little to show in the way of impressive abilities or popularity. His appearances in the MCU garnered significant interest in the character, to the point where he begins showing up in other installments without Strange himself. He also ended up ascending far higher in rank than his comic self ever did, as he ends up becoming Sorcerer Supreme during the five-year Snap interim.
Live-Action TV
Daredevil (2015)definitely did this for the titular character. While Daredevil has always been a well-known Marvel hero, he was never quite able to break out into mainstream comics like popularity of heroes like Spider-Man or the X-Men. After the acclaim of the Netflix series, his popularity has skyrocketed, with his series being considered one of the best Netflix shows. Daredevil's role in the Marvel TV-verse has even been compared to the way Iron Man ended up kicking off the MCU. It's no wonder that he was the first character from the Marvel Netflix shows to be introduced in a proper MCU film, specifically No Way Home. It's quite telling that his upcoming series on Disney+ is revealed to come with 18 episodes, by far the most episodes of any Marvel Disney+ show so far.
. Then Iron Man came out and made Tony Stark as synonymous with Marvel as Spidey and Thor and Captain America: The First Avenger did the same with its titular heroes and by the time of The Avengers (2012) they along with Black Widow and Hawkeye had catapulted into the mainstream and became the Marvel superhero team. Before then, the X-Men (and to lesser extent the Fantastic Four) had always held that title among readers and viewers. Thanks to the films, Iron Man, Cap, Thor, Black Widow and Hawkeye's comics got more recognition and sales than they’ve had in decades.