My favourite Avengers storyline is Civil War, but it would be impossible to bring to screen, however amazing it could be. It is far too complicated and really doesn't make any sense if the US are not filled with dozens of superheroes, which is not about to happen any time soon in the MCU.
I think the best one to make a movie about would be an Avengers V X-Men kind of event. Two teams of relatively equal power going toe to toe would be fantastic from the entertainment point of view, but what about I really like about that situation and Civil War is that each time both sides could be said to be equally right, or at least to have a valid case. No easy Manichean dichotomies between good and bad guys. I would however retain the right to bring up a bad guy at the end that the two teams would have to unite to defeat ; Onslaught seems like the obvious answer. Again though, this is an impossible solution, since Sony (evil, evil Sony) maintains the rights for the X-Men.
So my final answer is War of Kings. Some cosmic elements have already been introduced in Guardians of the Galaxy and GotG 2 could further our knowledge of the galactic MCU. The Inhumans are to appear shortly in their own movie, after having been presented in Marvel Agents of SHIELD. WoK would therefore nicely gather all different teams (assuming the rest of the Earth heroes eventually get into the Avengers) to fight a cosmic enemy, like I imagine Infinity War will do. Besides, in the comic book version, Vulcan is just badass, and I would really like to watch his battle against Black Bolt on screen as they are surrounded by the rest of the characters.
That being taken care of, The Vision does change his density in AoU. It's subtle, but sometimes when he is ripping apart robots, you can see his arms turn ghostly green as they go through his victims before becoming solid again.
As for the SW, that's complicated because her powers are never really completely defined. At some points she is basically able to do anything she wants, so restraining her abilities to a couple of easily understandable ones makes sense to me, though I would agree the ultimate choice wasn't necessarily the best one. I just have trouble understand what on earth the red stuff is supposed to be.