Including Shang-Chi, which is mostly the same generic origin story we've seen ten times before, in the poll instead of The Winter Soldier is baffling to say the least.
Imo cases can be made for the following:
Iron Man: Started the franchise, set the tone for the following movies
The Avengers: First successful team-up movie, as pivotal as Iron Man for kicking off the franchise, huge impact on pop culture
The Winter Soldier: Thematically the strongest entry
Black Panther: Cultural relevance, Oscar recognition
Infinity War: Culmination of the Infinity Saga and therefore of the biggest franchise of all time, huge impact on pop culture
Endgame: Conclusion of the Infinity Saga, held fondly due to its great fan service