Now THIS is a tough match-up! If we were going by most-improved, I'd go with Ragnarok, because that was miles above the other Thors. However, it's not most-improved, and Infinity War, as much as I love it, will inevitably be affected by its 2nd part, so until A4 comes out, I don't have a final verdict on how I feel about IW on the whole. So now I'm stuck between TDKR and Civil War. I really did love TDKR, just...not as much as the TDK. So while a good film, ultimately, it felt like a step down.
Civil War gets my vote because it doesn't feel like a step down from its predecessor, as it has a more powerful 3rd act where it really sticks the landing. And because it beautifully completes the patriot-to-insurgent arc for Cap that M&M/The Russos wanted to tell and again manages to at least match its predecessor in quality, I feel its the most complementary film on the list to the trilogy its serving. That trilogy, imo, when put together, says pretty much everything there is to say about who Captain America is - past, present and future. If I ever want to show someone why I love Cap as a character, all I have to do is show them those 3 movies and my job is done. Even with TFA in there, it still feels like one complete journey/experience. And for that reason, while I'm sure a reboot is inevitably down the line, I think it will stand as the "definitive" take on Captain America for a very long time.