I watched Infinity War this weekend, and one of the friends who went with me (he's seen all the films and enjoys the comics as well) also said it's now his number 1 for the current MCU films.
I personally had only watched 4 films prior, so that makes my list easy
- Avengers: Infinity War
- Black Panther
- Doctor Strange
- Guardians of the Galaxy
- Captain America: Civil War
Of all the films I watched prior to Infinity War, though, I'm glad I watched those 4. Had I missed any of those, I would've been confused for sure (esp with Doctor Strange / Black Panther). If I had gotten a chance to watch one more film prior, I think it would have to be one of the Thor films since I don't know anything about him, really, or Loki outside of the memes.