You can skip Black Panther. If you watch all and understood every characters minute details, you may enjoy each scene and even could predict the next. Still they can change the story as they like This is about IW2.
Thor, Captain America and Iron man all series are must. These will get you emotionally attached along with Avengers 1 and 2 and Guardians of the Galaxy 1 and 2. Guardians should give you the details of Thanos. He is the main attraction of the film which Marver was teasing us with Avengers from 2012. Can skip spiderman, antman and hulk. But just need to have the idea of the characters involved.
Loki will die in the first 5 mins. Marvel promised to show who Thanos is in first 5 minutes. He kills Loki and make Thor watch I think. If you recall, Loki had got help in form of Chitauri from Thanos in Avengers.