It needs to be the immediate sequel, not a few down the line. Final Fantasy VI is a lot better than Final Fantasy I, but there games inbetween that gradually improved the series.
Biggest jumps in quality I’ve seen are:
Zelda Breath of the Wild - I was a huge fan of the series, but the amount of things improved in this sequel are so huge, it relaunched and made the series amazing again
Witcher 3 - Witcher 2 actually gets a middling to negative review from me. The third Witcher game improved almost as many things as Breath of the Wild did. It’s an incredible leap in quality in all areas.
I think Assassin’s Creed 2 is often on these lists and it is a huge improvement, but it actually feels more like a finished game while the first game is just a demo. I also find Assassin’s Creed frustrating to control and kind of un-fun a lot of the times, so I still can’t give it much credit here.
What other sequels improved tremendously on their predecessor?