Endgame no doubt. The most character driven film of the bunch. That's why it's so different. They could've easily squeezed in another obligatory, tropish action set piece, 2 main sets and 3 acts like the typical superhero fare, but I'm glad they didn't. Instead they drive home the reason why it is we care so much for these characters and when it builds up to that crescendo where stakes are at its highest, it just works beautifully.
I hear some say Hulk or Thor were done wrong or never got their moment.. I actually didn't care for that. I actually found them more lovable here than ever before and that's again due to character work and genuine moments presented to us.