Neither AoU or Ant-Man are the best Marvel movies, but I agree that Ant-Man is definitely one of the more fun movies. And I reeeeally want Michael Peña to come back for another movie because he was one of my favorite things about it.
Having rewatched it, I feel like AoU was basically a set up for Civil War and introducing The Vision and that's kind of it. I felt like Ultron was pretty flat, but not because of Spader (who was awesome at voicing him), but because he didn't get a good enough set up for why he was doing what he was doing. I dunno. The other movies have flowed better and that one didn't. It's still a good hero movie, but the other Marvel movies have been so good it just sticks out I guess.