A Nightmare On Elm Street:
-the original is a ("the") classic, scary and surreal.
-i did eventually see the second, which is alright, but i skipped it for a long time because of its reputation.
-i actually did something i rarely do now and watched the 3rd (Dream Warriors) before the 2nd. this one is very good, great setting, decent scares.
-the first one i ever watched was the one that was new at the time, Wes Craven's New Nightmare. i've been meaning to rewatch it because it builds on the previous films. it did what the director did later with Scream as a metacommentary, which horror as a genre does anyway. it's scary and funny. this and the original i consider my favorites.
-then there's Freddy Vs. Jason. i'm sure most reading this have seen it and have their own opinions. this benefited from seeing on the big screen. i found it very fun and would like to see it again.

Friday The 13th:
-i've seen the first 4. all fine for what they are. this series/character/setup just isn't as clever as the freddy movies nor as beautiful. if the idea of seeing adults playing teenage campers getting slashed up for laughs doesn't repulse you, these fall somewhere in the realm of "good movies."
-Jason Goes To Hell i had to see. it's part 9, and i saw it before parts 2 through 4, mainly because i remember the video box so vividly and loved the premise. i don't put it anywhere above the previous parts.
-Jason X is fucking ridiculous. i found this easy to like because there's just no way to take it seriously, and the change of setting is at least appreciated.
-see comment above re: Freddy Vs. Jason.

Halloween:
-the original is a stone cold, influential classic. it stands above even other horror classics.
-the 2nd, which carpenter had a lot of involvement in but did not direct (except for 2nd unit!), i also liked.
-Halloween III: Season Of The Witch i put off watching for years because it's overall unpopular and doesn't have michael myers. i think the idea was just to have spooky movies under the Halloween banner. i enjoyed this quite a bit actually. if you don't mind that it's a detour, it's worth checking out.
-again probably like a lot of people here, i saw Halloween: H2O in the movie theaters. to date, i haven't seen anything between this and the above. for what it is and its era, i liked it.
-i also saw the rob zombie remake even though i am not a fan of remakes of horror films of relative recent vintage and/or ones that follow so quickly on legitimate sequels. i was surprised how much i enjoyed it even though it has the director's nastiness and brutality that don't jibe with the style of the original, but that's what remakes do, they offer another take. i've been meaning to see the second part.

The Omen:
-the original is considered a classic. it's the only essential one in the series.
-the 2nd and 3rd are pretty good with some dumb parts, and i stopped there.
-i actually ended up seeing the remake first, as a date movie, and i enjoyed it. in hindsight, i don't know that it really adds anything, but i remember liev schreiber being good in it.

Poltergeist:
-the original, again, is another classic.
-part 2 manages not to fuck up exactly. it's worth checking out if you liked the first but isn't essential. you could end the story just with the original movie.
-it would be a huge stretch to call the 3rd movie good. it's notoriously bad. there is enough going on in it to be fun to watch, and the behind the scenes story is fascinating (and sad). you'll know pretty quickly whether it's for you.

Hellraiser:
-the original is beautiful,
-so is the sequel,
-and the third i thought was pretty good. i did stop there. if you like surrealism, these have it and are pretty scary.

The Evil Dead:
-the original is low budget and enjoyable but was rightly "remade" as
-part 2, which is awesome.
-Army Of Darkness is good, a definite departure from the previous, and is widely considered a classic. i'm not quite as big a fan, but it's a fun ride, literally full of quotables and memorable gags.

Phantasm:
-first is very good,
-second a little less good but worth watching,
-third was fine for me to end on and be satisfied with the ones i watched.

Saw:
-i remember seeing the original in the movie theater and thinking it was so inventive, gory, and hard to take my eyes off.
-i saw the 2nd and 3rd parts on cable eventually and didn't really see the point. they were enjoyable enough, but it's more of the same, not unlike the first sequels to Friday The 13th. i personally am not someone who needs to see the same kind of story over and over. i wouldn't mind seeing the rest of the franchise eventually but am in no hurry whatsoever.

Alien:
-these are sci-fi so i won't go into detail, but they are as much horror IMO. excellent series.