I watched all of these in sequence about a year ago, throughout a week, not in one sitting of course.
I'm a huge horror fan, so I tend to find something in all the films that I can enjoy. One thing that always holds these films together is the fact that, regardless if whether you enjoy them or not, they all come off as quite scary, or at least have moments of being scary. Pinhead is just genuinely spooky, and the fact that he (Mostly) acts on duty as opposed to personal drive, makes him all the more frightening. "The BOX...you opened it, we came"....
As White Man said, the third one is probably the worst, if only because Pinhead turns into a typical "I'm taking over the world, for no reason" type villain, as opposed to a demon that simply does his job. The cenobites are also fucking ridiculous in it, even so, if you really dig the mythology/characters, you might enjoy it, but probably won't. For some reason, this film seems like it was the most influenced by the other horror movies of that time...Pinhead is even funny on an occasion or two.
DO NOT LET PART 3 DETER YOU FROM PART 4. Shocking as it may sound, Hellraiser 4 is actually a return to form. It's a pretty interesting mix of sci-fi, horror, and period piece. Delves into the mythology of pinhead, has some cool cenobites, and doesn't have the ridiculousness of 3.
Part 5, Inferno, is also definitely worth checking out if you're a fan of the series. It's actually a mix of detective fiction/horror/psychological thriller...Pinhead isn't in it all that much, but the villain that they introduce is damn near just as scary.
Part 6, Hellseeker, is a little bit like part 5, but closer to Jabobs Latter. I've always felt that part 5 and 6 sorta go hand in hand. Their very different in tone from the other films in the series, and can almost stand alone, but still quite scary and they somewhat go against genre type by focusing on two male victims.
Part 7, Deader, is kind of interesting. From what I understand, it was not originally supposed to be a Hellraiser film, and it really does show. It was obviously supposed to be a cult film of some sort, somewhere along the line, somebody must have secured some rights or something because Pinhead sticks out like a sore thumb. Even so, I believe that it was one of the more frightening entries in the series...honest to god, their is a part in the beginning where a person with a bad heart can have a cardiac arrest if they aren't careful.
Part 8, Hellworld....lol. If you're going to skip one, skip this one, as it's the last in the series before they do the remake, and really feels like a modern, run of the mill, shitty teenage sex horror. It delves into the "scary computer world" type stuff that you see in movies like Feardotcom and Stay Alive. If you really need motivation to see it, hell, it does have Lance Henriksen, but even he can't save this.
All in all, when matched up against A Nightmare on Elm Street, Childs Play, Halloween, and Friday the 13th, I'd say it ranks a close second to the Kruger films as far as being a horror franchise that wore its welcome. As for actually being scary, it's definitely the top of the list far as I'm concerned.
