This one is very close for me as several of the Phase One movies are strong contenders as are Guardians Vol. 1 and Spiderman but, at the end of the day, I'm going to go with the first Avengers. The reason for this is that it was such a spectacle a first-of-its-kind with the ensemble cast that had been built up over the course of several movies. I remember a lot of people were wondering if the quality would be there in trying to pull off a movie with such a large cast and so many leads, but I think most who saw the film would agree that it was a success. It wasn't the best as far as plot, but it was enough to showcase each character. Also, while I didn't feel Loki was the strongest villain, using him was smart in that the audience was familiar with the character already; so, the film really just had to focus on the ensemble and the spectacle of it all. Again, it may not be the best all-around movie, but I think The Avengers is in on top for me because of what the movie pulled off, and I remember leaving the theater feeling excited and wanting more.