Scream 2. Especially in terms of its deconstruction of the genre and self-awareness. It's never really been done better since.
Also Aliens. You can argue all day whether it was better or worse than Alien but it's still horror and it's still a great movie.
It's really really good and it definitely holds up well and I need to rewatch it because it was actually really prescient in depicting how quickly we now turn trauma and tragedy into spectacle. (It opens with a movie theatre playing an in universe movie based on the events of the first film).
Day of the Dead and Dawn of the Dead. I'd second Scream 2, Nightmare on Elm Street 3, and Evil Dead 2 which other users already suggested.
Nightmare on Elm Street 2 and 3
Texas Chainsaw Massacre 2
Friday the 13th part 2
The Purge: Anarchy
Devil's Rejects
Does aliens count? It's a sequel to a horror movie but not necessarily a horror movie itself
