I agree that Captain America: The Winter Soldier is a great film, but I consider The Avengers (2012) to be the best film in the MCU so far.
It's a classic and is superior to TWS because it's a full fledged comic-book movie; it features out-of-this-world characters, such as the Hulk and Loki, and fantastical elements, such as the Tesseract. On the other hand, The Winter Soldier could pass for a typical, albeit well-made, political and action thriller with a few sci-fi elements due to its grounded cast of characters and its grounded plot points; there's nothing fantastical or particularly comic-bookish about it.