For me IMHO:

1. Original Star Wars trilogy has been ruined by the Prequels.
2. Matrix ruined by Sequels
3. Aliens partially ruined by Alien3

Discuss

Matrix 3 absolutely destroyed Matrix 2 for me.
Same for Pirates 3. It was so bad it just shat on everything that was established in Part 2.

Sam Raimi pooped on his audience with Spider-Man 3.

edited to add (accidently hit 'save changes' instead of 'preview changes'):

It's hard for me watch what was built by SM2 today and just know how the next one effed it all up. Much the same can be said for the X-Men series. X2 was building to an awesome finale but when Singer left so did the mastermind behind the whole saga.

X-Men 3 (aka "X-Men: The Last Stand") partially ruined X-Men 2 (aka "X2")

Terminator 3: Rise of the Machines partially ruined Terminator 2: Judgment Day (and this ruining is continued by the TV series)

And the ruining of two separate franchises (Alien and Predator) is being done by the Alien vs. Predator movies. When speaking of the fans of these franchises, the tagline "Whoever wins... We lose" is all too true.

The good movies are still on my shelf, I just ignore the crappy sequels. Nothing has been ruined.

T2 kinda ruined The Terminator for me. Gave it a happy, cheesy ending and basically showed that SkyNet could send endless terminators back in time and for some reason didn't.

And then Singer went and took a dump on the Superman franchise. Although, III and IV beat him to the punch.

HOSTEL!!!Hostel 2 was one of the worst movies I have ever seen. Plus, after I saw it my penis hurt for about 2 hours.

100% agreed

I've never really bought into the idea that a sequel can ruin another movie...or rape my childhood...or any of that nonsense.

Amen to that. It's nonsense. Don't enjoy the sequel, don't watch it again. Problem solved.

The only example I can think of where a sequel killed my respect for the original was Shrek 2, one of the most overrated movies of the last decade IMO.

None that I can think of. Each film stands on its own merits.

QFT

Leonard, Parts I-V

The Star Wars original trilogy was not ruined by the prequels, ti was ruined by the edits that George Lucas made for the special editions and the DVD releases.

... and easily forgettable. Hence the original still standing on it's own.

Ditto. Thread over.

exactly. i take the ones i love on their own merit, if i like their sequels, then so be it, if not it doesn't take away from how much i loved the first or predecessor.

^OK but what about the movies that were shot back to back like PIrates 2/3, Matrix 2/3, Lord of the rings/ Kill Bill (not that I am debating these are bad, I love them just using them as an example or something thats really one story split into multiple films.)
I really liked Pirates 2 but I did not like Pirates 3 which continued the story of Pirates 2 . Same with Matrix 2/3 I loved Reloaded but Revolutions was horrendous.

Highlander. There should have been only one.

LOTR -- the second two films with all their battles and warfare really killed the "bunch of people walking around and talking a lot" mood from the first one.

American Psycho 2 --- Horrible movie that confirms Bateman killed people. WTF?

Star Wars Episode 1. The other 5 films were unnecessary and ruined it.

This is the correct answer.