I really like Winter Soldier but I'd put it as second or third best. The stakes are fairly low, and its got a lot of slow moments. To me peak MCU is probably always going to be Infinity War, just for the Empire Strikes Back factor - the heroes FUCKING LOSE and not in a small way, in a massive, everything-is-fucked way and we have to wait a full year to see how it resolves.
Jesus Christ, how often does that happen in a summer blockbuster movie. There are arguably also only two slow scenes in the entire movie, Wanda+Vision and some of the Wakanda stuff. Thanos is probably also the best comic book villain next to Ledger Joker, Winter Soldier is great but I dunno if I'd really call him a villain in that movie, its really old man Redford and Hydra.