Recently, Disney and Marvel compiled a recommended watchlist to help viewers recap the Marvel Cinematic Universe storyline before the release of Avengers: Doomsday. This official list includes 14 movies and two seasons of Loki, totaling 2,475 minutes, or 41 hours and 15 minutes.
Moreover, Disney suggests going back to the year 2000, as characters from the old X-Men universe will appear in Avengers: Doomsday. Therefore, topping the list is the original X-Men starring Patrick Stewart and Ian McKellen, followed closely by X2. After that, the watchlist jumps directly into the MCU (Marvel Cinematic Universe). For Phase One, you only need to watch Captain America: The First Avenger and the first The Avengers. Then Marvel recommends skipping straight to Avengers: Infinity War and Avengers: Endgame.
The real marathon begins with the Multiverse Saga. Loki holds a core position on this watchlist and is the only series officially included in Marvel's must-watch list. The two seasons combined account for nearly a quarter of the total runtime. If you want to follow the chronological order of events more closely between the two Loki seasons, you should watch Shang-Chi and the Legend of the Ten Rings, Spider-Man: No Way Home, and Black Panther: Wakanda Forever after finishing Season 1, before returning to watch Season 2 of Loki.
The final part of the watchlist mainly consists of Marvel's latest batch of works, including Captain America: Brave New World, Deadpool & Wolverine, Doctor Strange in the Multiverse of Madness, Thunderbolts*, and The Fantastic Four: First Steps.
However, the watchlist is missing a heavyweight contender, Spider-Man: Brand New Day. As Marvel's latest MCU film, this movie will surely serve as important background before watching Doomsday, but it is currently not included, likely because it is not yet available for home viewing via streaming platforms.
The final order of the watchlist is as follows:
X-Men
X2
Captain America: The First Avenger
The Avengers
Avengers: Infinity War
Avengers: Endgame
Loki Season 1
Shang-Chi and the Legend of the Ten Rings
Spider-Man: No Way Home
Black Panther: Wakanda Forever
Loki Season 2
Captain America: Brave New World
Deadpool & Wolverine
Doctor Strange in the Multiverse of Madness
Thunderbolts*
The Fantastic Four: First Steps