Iron Man (2008) - A solid, if not predictably crafted Origin Story that took a superhero genre plagued with Fantastic Four-esque films and made it "not awful".
This movie is held up if not solely by the performances by RDJ and Jeff Bridges.
The Incredible Hulk (2008) - The former "worst of the bunch", this cheap-looking, boring film strives for a different tone than any other Marvel film since. And I can appreciate that, if nothing else.
Iron Man 2 (2010) - The first time Studio intervention broke a film in recent memory.... but it shall not be the last. This movie had too many uninteresting, unrelated things happening that turned the narrative in to a game of scattegories.
Thor (2011) - A completely forgettable, lowest common denominator film that doesn't strive to do anything impactful, nor compelling. Fight scenes are visual blue-soup and the characters are devoid of any type of tension when they exist solely to crack jokes. This movie is driven if not solely by Tom Hiddleston and Anthony Hopkins.
Captain America (2011) - A watered down attempt at a war film that hits neither grit nor enjoyment. This film finds the time for a 4-minute dance number but for some reason plays all of its action sequences through montage. The film is poorly paced, is devoid of all character and charisma, and exists purely as a stepping-stone to get to The Avengers.
The Avengers (2012) - A well-written, slow building film that has a solidly defined 3-act structure that paces the film well. The only drawback is that this may be the most blandly shot film in the MCU.
Iron Man 3 (2013) - Mismarketed. Audiences were promised a deconstruction of Tony Stark's character in the face of having to make horrific decisions that impact the hero on an emotional level yet unprecedented. What we got is a low-stakes quip-fest that removes any sense of character continuity and suspense by having each situation undercut by the stupidity of it's genre.
Thor: The Dark World (2013) - I'm going to be entirely honest with you- I saw this movie and I cannot remember a single thing about it. It left my brain quicker than I could commit any of it to memory... probably done as a self-defense mechanism because nothing about this film is engaging.
Captain America: The Winter Soldier (2014) - Finally, filmmakers that understand that when dealing with a bland character- the best thing to do is to place said Saltine Cracker in a world of which undermines everything that they believe in to create a situation where the heroes of the past can no longer exist in the world we've created today. I don't believe this film qualifies as a "Super Hero Film", it is merely an Espionage film with a Super Hero in it. It is, in my opinion, the best that Marvel has to offer.
Guardians of the Galaxy (2014) - Let the quip-times roll. A movie that was an enjoyable enough experience the film time around- that somehow gets worse and worse every time I see it. When a film trying to subvert expectations becomes the expectation- there is really nothing else for the film to stand on.
Avengers: Age of Ultron (2015) - The Iron Man 2 syndrome multiplied tenfold.
Ant-Man (2015) - Are you ready for another remake of Iron Man? The only thing that makes this film enjoyable is how Peyton Reed's (Edgar Wright's?) directing style is a more noticable presence than on any other Marvel Project. The ensuing heist film is concise and well-executed.
Captain America: Civil War (2016) - The Avengers: Let's try this one more time. The only issue with this film (other than it being visually uninteresting and peaking in the 2nd act) is that this film constantly tries to undermine itself. Nearly every moment that should be dramatic is undercut by a quip. You want suspense? Nope. Quips. You want hard-hitting emotional repercussions? Nope- this is Marvel, we can't kill anyone.
Doctor Strange (2016) - This was the first one that truly insulted my intelligence. A remake of Iron Man so predictable and cliche that not even the actors performances can shake me of the fact that I've already seen this film before.
So like... the point I'm trying to make is that...
The Winter Soldier is great.
There are a few that are decent.
The rest are just bad.
So if you ask which Marvel film is the weakest- my answer would be:
"They're all too weak to really differentiate THE WORST of the worst".