The first Avengers movie succeeds on so many levels. Nowadays, seeing a group of Marvel characters in one movie seems natural, but at the time of this movie, it was completely unheard of.
The original six that start off The Avengers are all great in this movie, and work off each other very well, whilst also having great moments of their own. This movie is really cleverly written, with lines I love with every rewatch. The action, and how the characters use their powers on their own and in a team, are very creative and well done.
But what this movie succeeds at the most, and that none of the other MCU movies have failed to top is the third act. Virtually every single moment from the third act is everything I love about this franchise. And when I witnessed the 360 shot of the team all together for the first time, I knew the Avengers had truly assembled and that this franchise was here to stay.