Delete the MCU!

William Hurt returned as "Thunderbolt" Ross in Captain America: Civil War. So I guess that's Marvel Studio's way of saying, "Yes, The Incredible Hulk still count as canon."
That said, it's still the most disposable film.

Can't drop Ironman. It's the start.
Can't drop Ironman 2. That one introduces Black Widow and sets the precedent for how Tony Stark deals with personal problems: in the worst way possible. It's also the start of him pushing those closest to him away when the going gets tough.
Can't drop Thor. It introduces the interstellar/interdimensional aspects of the MCU, which later films like Guardians and Doctor Strange would expand on. And Loki.
Can't drop Captain America 1. That one introduces the Tesseract, the first of the Infinity Stones, and introduces the fact that Earth has been visited by otherworldly forces before. Plus, Chris Evans' abs.
Can't drop The Avengers. That one formally introduces the Infinity Stone arc that's gone onto the dominant the whole damn thing. Oh, and Thanos.
Can't drop Ironman 3. That one is where the paranoid, obsessed-with-protecting-everything-and-everyone Tony from Age of Ultron and Civil War was born.
Can't drop Thor 2. Infinity Stone business. And more Loki.
Can't drop Captain America: Winter Soldier. It introduces Falcon, brings back Bucky, and sets up everything that is to come politically within the narrative.
Can't drop Guardians of the Galaxy. It expands on the outer space part of the MCU introduced in Thor, and they're going to continue expanding out into it. Plus, another Infinity Stone. And sweet 70s tunes. And Howard the Duck (this is obviously the most important part).
Can't drop Avengers: Age of Ultron. It introduces Vision, Scarlet Witch, sends Hulk off to Ragnarok, sends Thor off on the journey that lands him in Ragnarok, and shows us for the first time just how Tony's paranoia and obsession is REALLY starting to strain his sanity and friendships. Plus, another Infinity Stone. And the creation of Ultron and dropping a city out of the sky are among the final straws leading to The Accords. ...That is a sentence I typed, and its accurate. Comic books are weird.
You could drop Ant-Man at this point, but its still less disposable than TIH.
You can't drop Captain America: Civil War. It introduces Black Panther, Spider-Man, The Accords, and the film ends with the Avengers split up and at odds with each other. Plus, Cap, Falcon, Bucky, Wanda, Hawkeye, Ant-Man, and Black Widow are all officially outlaws. Earth is more vulnerable than it ever was before with its mightiest heroes in-fighting instead of standing together. This is what's going to allow Thanos to take both Infinity Stones being held on Earth with such ease. Plus, Black Panther will likely go on to be one of the figures who assembles The Avengers in future film, alongside Doctor Strange.
You can't drop Doctor Strange, because now they're REALLY expanded on the multidimensional angle of the MCU, introduced Doctor Strange. Marvel was smart in bringing in both Strange and Ch'Talla with Downey's swansong coming up soon. Oh, and there's another Infinity Stone.
Can't drop Guardians Vol. 2. It introduces The Sovereign, and by extension: Adam Warlock. It also introduces The Watchers, even if it is during a comic relief moment. The fact remains we know they exist in the MCU now. And lastly, the original Guardians of the Galaxy are brought in. And, of course, the linchpin that holds the whole thing together: Howard the Duck.