Probably Colossus before the recent brainwashing incident.
Powerhouse character, sensitive artist, a genuinely good man. Deserved better than the crappy fake marriage. Deserves better than the "brainwashed by his brother" storyline.
When I was a kid, reading the original Avengers series, I really liked The Vision. Cool story, cool power, can fly.
A recent entry, but I adore Ms. Marvel. She’s very much like Peter Parker when he was introduced—teenager, dealing with real-world problems as well as superhero problems—except that she’s also a Pakistani immigrant living in New Jersey, so she has culture clash issues to deal with on top of the rest. The best parts of her story are when her superhero life intersects with her family/friends life, just like Peter Parker back in the 60s.
Not a fan of the new “she’s actually a mutant and being drawn into the Krakoa X-Men storylines”. I love the Krakoa X-men storylines but I think they don’t fit with her very grounded character.
Doreen Green aka Squirrel Girl. I love a lot of comics but I seriously looked forward to every single issue of The Unbeatable Squirrel Girl (2015-2019). Ryan North and Erica Henderson became one of my favorite creative teams for making something that is so hilariously clever at times and just absolute fun all the time.
The North/Henderson team also did the graphic novel The Unbeatable Squirrel Girl Beats Up the Marvel Universe.
That's a really tough question that can change based on the writer or my own whims!
Right now I would have to say Mirage: There's something I love about a character who starts out as an insecure outcast that's angsty, distrusting, and even resentful to those around her, only to grow into a strong and compassionate leader who's always looking out for her team.
Probably Spider-man?
Groo the Wanderer if he counts as a Marvel character. https://en.wikipedia.org/wiki/Groo_the_Wanderer
If not then Peter Parker's Spiderman. Yes it's cheating, he is, by a wide margin, the best selling comic book character. There's a good reason he's so popular. The books are good. I don't know why the movies haven't picked up on this but one of the best things about the character is that you get to see him grow up and develop both as a superhero and through his transition from teenager to college student to working adult. He doesn't start out as a fully developed hero saving the world at every turn. You see him making mistakes and you see him discovering and figuring out how to overcome his flaws. As he grows up and becomes more competent as a hero he never becomes the perfect fully developed superhero a la Superman or Batman. As he figures out one part of his life he transitions into another and has a whole new set of challenges to overcome. And despite never becoming that mythic figure he's still an Avenger. Despite just being your friendly neighborhood Spiderman, he's still fought and defeated the biggest and baddest of the Marvel rogues gallery.
Elf With a Gun
