Mentioned Civil War because it is essentially an "unofficial Avengers" movie.
Guardians....i'll leave that one alone.
. Plus, GOTG2 was a drop-off.
I get what you are saying though.
Captain America (WS and CW), Black Panther, and Infinity War.
Arguably, the most critically acclaimed MCU movies.
Look at the subject matter though. You couldn't just throw a guy like Whedon on those projects. A lot was at stake with each movie. They had to raise the bar given the material and expectations.
Captain America's Civil War: Had to follow-up Winter Solider, introduce and setup Black Panther and introduce and setup MCU Spider-Man for the upcoming movies. They accomplished ALL of that. That required more than the usual "Hey, the gangs all here" theme of the 1st 2 Avenger movies.
Black Panther: Needs no explanation for this. This was in demand for decades. Marvel couldn't fumble this or there would have been trouble.
Infinity War: Again, needs no explanation. This is what the MCU was created for and they couldn't fumble this one either.
Marvel learns and pays attention. Over time they get better, and one of their best attributes is knowing how to approach each project accordingly. It is not a one size fits all with each character. That is where DC stumbled.