Captain America, easily. His series has had 3 films, and they’ve all been good to great. Avengers, the first one is one of the best super heroes films ever made, maybe even edges out the The Winter Soldier, but the second one while still fun lacks rewatchability and is worse than any of the Cap movies, so even if they averaged out the same, Cap still wins on consistency. If Infinity War is really good, it could make it interesting, but the other two just don’t really compare. Anyway, here’s how I feel about them:
Iron Man: B+ A really solid film and a great establishment of the character and tone of the universe, but forgettable villain keeps it from being an A.
Iron Man 2: D+ Mediocre at best, too much focus on world building, not forgettable but downright bad villains, messy plot.
Iron Man 3: C- It’s a well made movie, but the “twist” craps all over Iron Man’s, and one of Marvel’s overall, best villains, but I give them props for having the balls to do it. I really didn’t feel any stakes at the end with a bunch of empty suits fighting nameless bad guys.
Thor: B I liked that it was a character study to introduce us to his humanity first. Loki is a break-out villain. Could have used a little more action
Thor: The Dark World: C Again, fun with Loki, but too much plot convenience and another forgettable villain
Captain America: The First Avenger: B I probably like this more than most people do, but I went in expecting something really bad and I find it’s campiness to be charming. Red Skull is one of Marvel’s better villains.
Captain America: The Winter Soldier: A Love the tone, love the spy thriller aspect, great villains if just a hair under used. Lots of great action and great character moments and a well executed twist. All of my complaints are minor.
Captain America: Civil War: B A whole lot of fun, and great introduction to Black Panther and a large ensemble in general without losing focus on Cap himself. The Airport scene is a great but lacked stakes imo. Some of the plot points were too convenient. Another unmemorable villain (but forgivable since he’s more a plot device than a villain)
The Avengers: A Possibly their best overall film, but it’s real close Winter Soldier. More fun with Loki, and a blast to see them team up for the first time. Really felt Hawkeye was shafted (especially as we’d only been barely introduced to him) and a few minor plot issues, but probably the most rewatchability of all of their films.
Avengers: Age of Ultron: C+ Lots of good action, but largely a retread of the first one in overall story. Villain was decent enough but some pacing and story issues with more focus on setting the stage for phase 3 than paying off phase 2 where the previous paid off the whole of phase 1.