I haven't seen all of them, but I think each falls into their own category. Just like Star Wars and The Godfather are great in their respective genres, I think each Marvel movie can be good in its own field. In terms of plot, I think Guardians was typical (he even had the "Okay guys, we can either go home or go big" type of speech), but its characters were wacky enough to make up for it. It's not everyday you can make a raccoon and tree be best friends. Visually I'd say Doctor Strange was the best (due to the effects and other dimensions), but the villain (the magic guy, I can't remember his name) was weak and forgettable except for his purple eyes. However, the ending was really cool and out there (I'm talking about the time loop). That really did something else with an otherwise typical story. The Avengers had the best villain in Loki. Infinity War was a huge epic and "subverted my expectations", due to its ending and Thanos being the main character. Character wise, I think any Iron Man movie is the best, because Tony is the heart of the series.