The Lord of the Rings trilogy is an example. All three movies are epic and it's definetaley one of the best fantasy trilogies out there
Back to the Future is also a good example. Yes, BTTF1 is the best but 2 and 3 were still really good.
For franchises, the Harry Potter franchise. All 8 movies are pretty good, PoA, GoF and DH2 being the best imo. Fantastic Beasts is it's own seperate franchise so those movies don't count.