There hasn't been a lot of "why" so far.
Black Panther. It's the best directed, has the highest level of craft, and has the most emotional standalone story. Best music too!
Yeah, agreed on it having the best music, that soundtrack is fire. I'm not sure I agree on the "craft" part, because some of the CGI in that movie was questionable (that final fight, yikes), but in spite of that it's certainly one of my favorites, great movie.
I love the first Iron Man, and without it I really think we wouldn't have an MCU (particularly, of course, RDJ), but Guardians of the Galaxy takes this nod. Both 1 and 2 are great, but the first film just came out of nowhere for me and really established the zaniness of this series. If I had to pinpoint a favorite moment in the MCU, though, it would have to be the opening to Guardians 2. Mr. Blue Sky is just gold!
You're one of mine, Mr. Blue Sky is absolute gold and so is that whole scene. The ending is definitely the best moment for me, though.