Phase 1 was a nice intro, Phase 2 had a rocky start w/ IM3 & Thor Dark World....but my gawd Phase 3 is the prime of Marvel, imo....I could honestly select any one of those choices.
Civil War - Brilliant. Such a darker toned Marvel movie with such emotional depth with Tony and the tension between both sides seemed legit. A lot of twists and turns in this film. The intro of Spider-Man & Black Panther were epic and just created so much excitement for their solo movies.
Guardians Vol.2 - Loved this one. It still felt like the previous film, but new and fresh. Yondu made the film. Kurt Russell as Ego was so freaking awesome and such an epic villain. The sister rivalry storyline continued and developed so well.
Spider-Man Homecoming - After the TERRIBLE last three Spider-Man films, MCU fixed all the mistakes of Marc Webb's Spider-Man. It was a different take on Spider-Man, but still felt true to his origins. The addition of Iron Man / Tony in the film was a nice touch and made it truly feel like Peter Parker belonged in the MCU universe. Michael Keaton as Vulture was absolutely perfect. He was a great villain. Rather than the usual "we have to stop villain before he destroys the world" - it was a smaller story about Spider-Man trying to stop a thief (Vulture). Thats really it. That was true to the early comics of Spider-Man just tries to stop robbers and thieves.
Thor Ragnarok - Colorful, funny, light hearted, great villain - what more could you want from a Threequel ? Thor Ragnarok was so damn entertaining from start to finish. Jeff Goldblum, Karl Urban, and Cate Blanchett were perfect casting choices and made the film even that much more better. It was great to see MCU do a buddy adventure movie with two members of the Avengers.
Black Panther - One of the most emotional MCU movies ever. So much heart. Black Panther delivered on every level for me. It also had one of the best MCU villains of all time. The movie was so epic to me.
But my favorite of Phase 3.......
I think I'm the only one who voted for Doctor Strange, but DS was my favorite of them all. He was an interesting character and I think out of all the Phase 3 films, Doctor Strange had the best character arc. He went from cocky Tony Stark like character to a humbled mature wizard. It was the best arc for me. The relationship between him and the ancient one was very interesting and compelling. I can rewatch this one prolly the most out of all Phase 3.