...movies: the third act. Marvel just doesn't do villains very well and thus the big finale usually suffers for it.
IMO, the exceptions to this were Winter Soldier, Civil War and Homecoming. Winter Solider and especially Civil War had satisfying third acts because the "villain" was an established character we already knew and Homecoming was because Michael Keaton was just so dang good.
Tom Hiddleston's Loki has been great as well, but the third acts of the movies he appears in were weak.