Guardians of the Galaxy vol 1
For me all the Avengers movies are tied but if I tiered them:
Endgame
Age of Ultron
Infinity War
Avengers
The Winter Soldier
Civil War
Far From Home
Ragnarok
Homecoming
Iron Man 3
Ant man
Black Panther
Captain Marvel
Guardians of the Galaxy vol2
Dark World
Iron Man 2
First Avenger
Ant Man and Wasp
Thor
Incredible Hulk
As far as my personal watch order I usually start with Avengers but if I were uninitiated it would be:
Iron Man 2 (intro to Black Widow and War Machine)
First Avenger
Avengers
Winter Soldier (Falcon and Crossbones)
Honorable mention: Dark World
Honorable Mention: Ant Man
Civil War
Homecoming
Black Panther
Doctor Strange
Ragnarok
Infinity War
Honorable Mention: Ant Man and Wasp
Endgame
Captain Marvel
Far From Home