This is a 3-way tie for me.
Captain America: TFA
Just a very bland movie. I thought the skinny Steve stuff was infinitely better than anything after he turned into a super soldier. Yet, the action was lackluster (especially the final showdown) and it suffered from their rush job to get him to the Avengers.
The best part about the film was Hugo Weaving, whom they didn't give anywhere near enough material. So disappointed to know he'll never be back as the Red Skull. Honestly I would have preferred to see the period piece fleshed out over 2-3 movies. I say all this, yet ironically the sequel CA:TWS is my favorite Marvel film.
Thor: TDW
After initially being OK with it, this movie got worse with each viewing. Now I can't stand it at all. The opening sequence with Bor (ironically done by Tim Miller and Blue studios who are fresh off Deadpool) was the best part of the whole movie. Too much flat comedy and a third act that was horribly neutered.
That fact that there were 5-6 different guys who worked on the screenplay showed. Marvel really left me scratching my head. Damn shame because Chris Hemsworth is a freakin' superhero. He's being relegated to slapstick comedy. Thor is a character worthy of "Ohhhh sh--!" moments and Marvel has yet really deliver.
Guardians of the Galaxy
I'm going to take heat for this. But I couldn't stand the movie at all. I found it to be childish and I really wish someone else would've written the screenplay for the sequel instead of Gunn. I cringed during the Gamora/Nebula fight scene when Nebs literally tells her, "Oh well you're stupid!" Then there's Drax drunk dialing Ronan. I mean really??? I love Chris Pratt, but he tipped over into the void of obnoxiousness with all his "Bro!" antics.
The film's worst offense was over-stuffing the movie. I get it, they were trying to unspool a very large portion of the Cosmic MCU. The problem is that it birthed too much freaking exposition. "You're Drax the Destroyer!" "You're Gamora one of the deadliest assassins!" "He's Thanos the most fearest being in the galaxy!" Show me, don't tell me EVERYTHING.
Ronan ended up being wasted. Just like Huge Weaving with the Red Skull, Lee Pace did his best with what he could and they rewarded him with some silly dance off at the end. I thought Gunn was trying too hard to copycat Whedon's ending in the Avengers with Hulk/Loki. It was so much so that he did it twice (along with the Groot scene slamming around the Sakaaran).