My favorite was Black Widow vs Hawkeye in Endgame not only was that super emotional but the fight choreography wasn’t half-bad. Thor vs hulk was amazing in Ragnarok; though emotionally I would say Thanos vs either Captain America and Iron Man were really meaningful and fun to watch. To choose another one I also feel like I should mention Captain America and Winter Soldier in The Winter Soldier that was absolutely astounding. Abomination vs Hulk was also fun to watch.
I will say tho from what I’m hearing about Shang-Chi I think these are soon going to be easily outdone and I can’t wait to see it in theaters.