Let's start With Best to Worst: (the first 5 being the stongest(mostly) and the rest being the weakest First of all I want to begin with Halloween: parts 1,6,2,4,5, are the strongest and parts 3,7,8 and the remake are the weakest. Now A Nightmare on Elm Street parts 2,4,1,3, and 6 are the strongest parts 5 and 7 are the weakest. Now Friday the 13th parts 3,6,1,2,7, and Jason X are the strongest and parts 5,8, and Jason Goes To Hell being the weakest. Child's Play parts 1,5,2 and 3(too bad for Bride of Chucky) and finally The Howling parts1,3,6,2 and 5 are the strongest and parts 4 and 7 JUST PLAIN SUCK! Enjoy and Happy Halloween!

I liked Halloween 1,2,3(Despite it not continuing the series its a great story),4, and H20
Friday 1-5(all the rest were awful) I'm one of the few fans of part 5, I think it's the last sequel to feel like it's part of the series, they were creating one of the most interesting slasher film characters with Tommy, I remember seeing it in the theater and being excited for a sequel with Tommy as the new killer....Then part 6 comes along and suddenly Tommy is a stupid redneck.
Child's Play and Bride of Chucky are the only ones I liked.
Nightmare on Elm Street 1,2,3, and New Nightmare were the ones I liked.
The Howling 1 and 3(3 because it's such a great cheesy movie), all the rest I didn't like.

George Romero's "Dead" series - All of them decently strong, especially "Night" and "Dawn". "Day of the Dead" , to me is the weakest, but Savini makes up for it in the end.

"Friday the 13th" series - First four flicks are by far the best in the series, with 5 through what ever number it stopped on, simply embarrasing. (they're remaking the original from what I've heard)

"The Elm St. series" - Original is heads above the rest, the second one is decent too. I know a lot of fans enjoy the 3rd installment, but Freddy became more of comedian than the eerie evil child killer he was in the first one.
The rest of the flicks 'tongue my hole'

"Halloween" series - John Carpenter's "Halloween" is by far my favorite Horror/Thriller of all time along with "Dressed to Kill", but every sequel has done nothing but tarnish the original.
Halloween II was the best sequel and it has major flaws with Michael and the script
#1 Michael has shrunk by 2 feet!
#2 Michael has gone from boogeyman to walking zombie, and I think he must have seen that "Buggs Bunny" episode of "The Rabbit and the Turtle" and figured out how the turtle kept up!
#3 Michael and Laurie are not brother and sister, this was Carpenter's way of giving Michael a reason to still be after her

The Howling series best to worst.

The Howling 5 Rebirth
The Howling
The Howling 6 The Freaks
The Howling 7
The Howling 3
Sorry, but I haven't seen 2 or 4 yet so I left them out for now

Some other horror franchises best to worst:

Night of the Living Dead
Dawn of the Dead
Day of the Dead
Land of the Dead

Phantasm 1 and 2 are both great horror films.
Phantasm 3 and 4 are cheesy fun.

Alien
Aliens
(Didn't like the rest)

Demons 1 and 2
(Didn't like the rest)

Silent Night Deadly Night 1, 4, 3, 5, 2