Guardians of the Galaxy is better at ensemble casts than Avengers. I’m only comparing the first movies in each case, as I’m not sure if Age of Ultron did it better or if Civil War technically counts as an Avengers movie. I considered going over Suicide Squad as well, but since I haven’t seen it and have never had any desire to do so, I contented myself with mentioning it where appropriate.
They each have a fair-sized group of characters with distinct abilities, personalities, and histories. But Avengers makes the mistake of treating them like a lineup. Most are introduced in a scene almost divorced from the larger narrative. Recruiting is a not always the best way to show a team assembling. It is certainly better than Suicide Squad’s dumb Powerpoint slide intro card things. But in the case of the Avengers, these are characters who either had their own movie, almost had their own movie, or will never get one but should have done. Thor is the only one whose introduction into the movie is organic.
Conversely, in Guardians of the Galaxy, each of the characters comes into the story rather than being recruited into the movie. Although it’s worth noting that this film didn’t go the same route of movies tying in to each other, I don’t think it’s the reason that GotG worked better. It all comes down to telling the story in a natural way, rather than the committee-driven look of introducing characters rather than integrating them.
Another thing that helps is that GotG has a main character. Even Suicide Squad had Deadshot. Starlord is the heart of the story, a stabilising force on the team (as much of a leader as they really have), and gives the movie an audience proxy. The movie isn’t Starlord Plus These Other Guys, either. He doesn’t get screen time or story focus instead of another character getting it.
An argument can be made that Nick Fury is the main character of Avengers, but if he is, then he’s a weak one. Too much of him is held back because he’s mysterious, and he is technically a supporting character for the ensemble, not a part of it.
Maybe this means that GotG is a better film overall than Avengers. maybe it doesn’t. That would need to take into account quite a few additional factors to determine, in my opinion.
Superhero comics are one of the places one can look for ensemble casts, though I can’t speak to their average quality. (Fantastic Four, cough cough) Superhero ensembles that have made it to film, at least in recent years, haven’t tended to be good ensembles. Big Hero 6 had a team in it, but it was a Boy and His Robot movie. Aside from the Teenage Mutant Ninja Turtles and that Roger Corman Fantastic Four… occurrence… in 1994, there wasn’t a comic book superhero team movie until Mystery Men.
Now there are a fair number, so it might be interesting to go over them and see how they did.