This is one of the best films I've ever seen, not just because it's a fun action film, filled with characters that I've grown to love, but because it was the first of its kind, setting the bar for other multiple-character franchises. It's a first, and because of this it absolutely has its flaws, but it's definitely a game changer.
Despite having seven lead characters (counting all Avengers plus Loki), everyone got at least a few moments of screen time devoted to their development. With seven leads, that's pretty amazing. We have the quiet moments, the little gestures and phrases that have been picked apart time and again by the fans that have granted a depth to the film that wouldn't have been possible if this were the first time we met any of these characters, and that's the real beauty of Avengers. It's a team-up flick, but it's a team-up of characters we've all been introduced to before. (Even if you hadn't seen any of the previous films, there was a sort of pop culture awareness around most of them by the time it was released that you'd be hard pressed to go in blind about anyone except the characters who didn't have their own film yet and were, however regrettably, less important because of it.)
Combine that with a well-loved director who isn't afraid to sucker punch his audience, and you've got a perfect mix for a film that doesn't just thrill audiences with fight scenes that are larger-than-life, but gets people to invest emotionally in a fictional world. Avengers was released more than two years ago, but the film is still being picked apart and very much alive in the hearts and minds of the fans because of Marvel's expert world-building. In every show, every film, you see threads that lead back (or forward) to other events in the Universe, and it's this sort of continuity that the fans love. They pick up on every little reference, read into every word of dialogue, every prop placement, every choice that the creative team makes, and this is what has made Avengers so successful. It stands up to that test, and even with ten films under Marvel's belt, there's still very few inconsistencies. (Especially when you compare it to the next largest Marvel franchise, X-Men, which has some major timeline problems around Wolverine.)
All in all, this is a film that should be in every collection because it's not just great, it's revolutionary.