Very much The Empire Strikes Back. A dark episode, marvelously handled. Nothing goes the Avengers way yet they still fight on, crack wise, and keep hope alive.
One of the biggest movies in history, but it is dependent on you having seen many earlier films to follow the details, not to mention know the characters. I would have thought such a thing could not be done. It shows how deeply ingrained this series already is in our culture.
RDJ/Ironman stands out here. His path and his interactions with the characters he crosses are the best, although the Thor/Starlord meeting is a gem. I don't have to recap what was good because it's the same good stuff you've seen in all the other films, only more so. This movie is unbelievably action dense, and I mean that in the sense that physical action is deeply intertwined with the dramatic action of moving the plot and characters forward. In most films you have action (bang, boom) then pauses to advance the setting (dramatic action) for the next bit of bang-boom action. Separating them opens you to the trap of pointless pyrotechnics or worse, extensive exposition. Not here. I would be willing to bet there are never more than 3 minutes of talk without activity and any explanations are given ether as a single line or in the course of battle.
Was there anything to dislike? Well, Peter Dinklage was poorly used. The Black Panther is still painfully sincere and haughty. I don't think Ruffalo or Johansen were at their best -- their lines didn't seem all that well delivered, and I still struggle with Cumberbatch's American accent. This is niggling.
I have argued before that blockbuster action films are this era's premier creative achievement -- like classic rock in the '60s, or auteur TV in the aughts. In the Blockbuster Action era, Infinity War, and Avengers 4 next year, could be the peak -- like Sgt. Pepper or Sopranos. When it's all over I may spiral into depression. I'm sure what will follow on will be watered down replacement heroes and race and gender stunt casting. With any luck I'll be gone by the time the reboots start.
I'm just delighted these stories from my adolescence have come alive so wonderfully at this point in my life. It's almost like a form of closure.