Even though many terrible horror movies have been made I'd say about 50% of the horror movies made are good, or at least decent, but their sequels almost always suck.
However, a number of sequels have been made that truly rival the original film in terms of quality and viewing enjoyment, a few of my faves being Friday the 13th Part 2, Evil Dead II and A Nighmare On Elm Street 3: Dream Warriors.
Friday the 13th Part 2 is the first appearance of adult Jason, Evil Dead II sets up the franchise nicely for all the Deadite fun to come, and Dream Warriors has the most creative dreams, and kills, in the series.
But why stop there when Wes Craven's New Nightmare had Freddy IRL hunting the creators of the series? And let's not forget Halloween 4: The Return Of Michael Myers, aka the flick that brought ol' Mikey back with a vengeance.
See 18 Horror Movie Sequels That Are Better Than The Original here