Might be a bit overdone, but I'll have to say Endgame, though Doctor Strange comes really close.
There was just something magical going to see Endgame opening night in theatres, seeing years of buildup, years of development, all come together in one three hour adventure. Seeing Cap prove he's worthy, seeing everyone come together to take on the Mad Titan, seeing Tony and Peter reunite, seeing Wanda body Thanos, seeing Carol destroy sanctuary, and seeing Tony take the ultimate sacrifice. It's far from the best of the marvel movies, but none of the other movies come anywhere close to giving me the same feelings I felt watching Endgame that day, not even Infinity War, enough to make it one of my all time favorite movies.
Doctor Strange was the movie that truly made me invested in the MCU. I had seen the movies, sure, but I hadn't really felt that much. The I saw Doctor Strange, and seeing a completely different world, of magic and mysticism, one where conventional wisdom means nothing, made me realize how much there really was to enjoy in the MCU