There are so few films were the sequel is actually good, partly I think because a sequel tends to be made just for cash. An exception is LOTR where I like The Two Towers best of the trilogy, but that had to be made--likewise the Harry Potter books- from the third film I think the quality and the acting is so much better.
But most of all Toy Story 2 and 3. Toy Story 3 made me cry, the first time a film had done that for many years and I think only the third time I'd ever cried in a cinema (the first time was Whale Rider and the second the first five minutes of Up!). But those films were a tear or two- I was in floods for probably the last half an hour of Toy Story 3 (then eleven year old sister sitting without a tear on her face whilst everyone over the age of 16 or so seemed to be at least sniffing). It's a work of genius and it deserved to win the Best Picture Award over King's Speech (I think)