Star Trek 2: The Wrath of Khan. Its predecessor is one of the weakest Trek films period, while Khan is widely regarded as the best of the TOS movies and remains everyone's reference point for top quality Trek movies. To be favorably compared to Khan is a success in its own right.
22 Jump Street / Austin Powers 2. While both of these recycle the jokes of the previous, comedic timing and delivery is greatly improved, and everyone just seems to be a lot more confident with the roles and having a lot more fun.
Mission: Impossible - Fallout > Mission: Impossible - Rogue Nation (while most of the franchise has some loose continuity, Fallout is a direct sequel to Rogue Nation). The stakes are high, the tension and sense of urgency rarely ever lets up, and Fallout manages not only to build upon Rogue Nation but draw elements and story beats from the rest of the series as well.
John Wick 3 > 2 > OG Each movie is great, but as the stakes become greater and the production value is slowly dialed up, more is revealed of the world's lore and the side characters get more to do.
Troll 2. While technically not a sequel for it doesn't have any of the same characters or even any trolls in it (I think the studio just had such little faith in the movie they declared it a sequel to guarantee at least a few ticket sales), Troll 2 managed to be a laughably bad movie while Troll is just a bad and utterly forgettable one.
Pirates of the Caribbean: Dead Man's Chest. Manages to one-up Black Pearl in almost every respect.