I genuinely think watching Endgame was the greatest cinematic experience of my life. It was 22 films and 11 years worth of build up paid off in the most perfect way. There was fan service for the film fans, fan service for the comic readers, and even fan service for the casual viewer who has seen the occasional MCU movie. The comedy was on point, the drama was heartbreaking, it is a masterpiece in film making and story telling. So that's why it's number one in my opinion.
It's also one of only three superhero films to crack my (open to change) top 10 movies, which also currently (and usually) includes Pulp Fiction, Trainspotting, In Bruges, Ragnarok, Infinity War, and Goodfellas. Last 3 spaces are open to several other contenders... But that's probably worth a new thread of its own.
Now that is one hell of a rambling post, thanks for getting to the end!