Let's face facts for a moment, there are a lot of Marvel movies out there.
The Marvel Cinematic Universe has taken the world by storm for nearly the last decade, films based on Marvel properties from other studios have been prevalent for almost twice as long with no end in sight, and that's not even counting all the other one-offs and TV movies that popped up back in the day.
That said, when you have many films of one category like this, there are bound to be a few underrated gems out there. In Marvel's case, an underrated film can be one that was serviceable, but overlooked; one that had a few small flaws that caused audiences to dismiss them, despite them otherwise being great; or one that was absolutely despised by fans and/or critics for one reason or another, but is actually quite good. Of course, it could also be any combination of those things.
There have been some irredeemably bad Marvel films over the years (Ang Lee's Hulk and Josh Trank's Fantastic Four spring to mind), but these should absolutely not be deemed as such. And if you rejected these movies the first time around, then maybe it's time to give them that all-important second chance.