I liked "Guardians of the Galaxy 2" much more than its predecessor. I know, I'm probably in the minority on that, but I thought it was a more accomplished piece of filmmaking, took more risks than the original, deepened its world-building, was a fuller and richer story and ultimately gave the audience more to attach themself to. The original GOTG was fun, but felt like a disposable piece of popcorn entertainment at the time. On future viewings it never got any better, but GOTG2 did benefit and improve with repeat viewings.
I thought "X-Men: Days of Future Past" was one of the finest comic/superhero films ever, made but I don't know if it counts in this list because its so far down and deep into the X-Men cinematic mythos that it isn't really a sequel. I suppose it was more of a sequel to "X-Men: First Class", which was a decent film itself, and is part of the second arc of X-Men films, after the first three plus "X-Men Origins: Wolverine" wave. How do we count sequels that are the third, fourth or later in the list?
"Black Panther 2" was far superior to its mediocre original, but the original established so much that could only be celebrated or compromised by an awkward and almost unneeded sequel. I don't know if many would agree, but BP2 did what couldn't be done, under the circumstances, and somehow succeeded masterfully.
Sometimes the original first films are procedural pieces that exist just to set up a franchise - that's their purpose and burden, to effectively function as "origin" films, to establish elements that allow the sequel(s) to really take flight and carry the true weight of the story. Marvel got pretty active churning out second-rate origin films just to launch characters into the larger Avengers/Infinity plot and then their own films and shows. I actually saw the Infinity films as a sort of weird new form of comprehensive genre sequel that branched, bridged and bound various Marvel films that came before. Little satellite films that laid down characters and world-building groundwork then letting the big motherlode Avengers/Infinity sequel take on the heavy lifting. "Avengers: Infinity War" alone surely outdid any Avengers film that came before, but then again we'd never have anything if it wasn't for the original. Sometimes you have to appreciate an original for what it is/was and how it established a franchise. For me there's no "The Godfather II" without "The Godfather", no "Empire Strikes Back" without "A New Hope" and no "Aliens" without "Alien". The sequels aren't better, they just ARE what they are, and sometimes they're part of an expansion of a great idea.