Rewatched while I was in Nashville. It was on TV and I didn't dare to change the channel.
Winter Soldier is most certainly Marvel's best film, and it's a stellar blockbuster apart from the limitations of the MCU to begin with. While in movies like The Avengers and even Captain America: First Avenger, the character of Steve Rogers was a little shallow at times, this film adds a whole new depth to his persona and a level of inner toil that makes the character far more understandable and sympathetic. This is definitely the best depiction of Captain America in the MCU thus far. Not to mention, the three major supporting characters of Falcon, Black Widow and Nick Fury are all pretty 3-dimensional too. Falcon is introduced really well, Black Widow takes on a whole new level of dimension that we'd only caught glimpses of in the past, and the character of Nick Fury becomes more flawed and human than he ever was before.
The thing, I think, that makes this movie the best in the MCU is that while the action is amazing and the VFX are great and the score is awesome, the main focus of the film is on the characters and their inner struggles. The friendship between Cap and Falcon is built up beautifully in a mere two or three scenes. The slightly flirtatious relationship between Steve and Natasha is great, too, with their opposing convictions challenging each other in a really interesting way. And then of course there's Steve's friendship/bromance with Bucky Barnes, which has spawned much insane fanart. But hey, for good reason - it's a great dynamic.