I already mentioned Friday the 13th Part 4, but I'd add part 2 and 3. I find part 1 a little overrated, and it also really lags with some slow pacing, for example an entire scene that shows Alice slowly brewing a pot of coffee. I get they were going for a slow buildup of suspense, but it doesn't always work as well as that sort of thing did in prior slashers like Texas Chainsaw and Halloween, or the Italian giallos. 2-4 really improved on part 1 and made a true art out of slasher movies. Sean S Cunningham was always better as a producer of horror films than as a director. He understood the business side of it, how to market, but I think he was average, at best, at the craft of directing--this really shows if you look into the making of the first film. He came up with a title before they even had a story or concept in mind, and he spent time marketing and raising funds based solely on his ability to persuade people that he was going to make the scariest film ever.
