Both Guardians of the Galaxy and Winter Soldier.
GOG personally, I feel has the best balance between comedy and action, all characters are incredibly charismatic, the set pieces/cinematography and CGI is fantastic. Theres very little to complain for me about it. Sure, it deviates a lot from the source material but it made me care about characters I didnt care for one bit. Came as a surprise tbh.
Winter Soldier on the other hand has (for me) the best plot, tone wise feels unique compared to other MCU films, it has a very strong screenplay, the narrative structure of the film is flawless. The action is fantastically choreographed/edited and mantains the special effects quality and comedy that distinguishes the entire MCU. Just very enjoyable.
In no particular order, I'd put Civil War, Ragnarok, Iron Man (1) and Infinity War after those 2.