Jaws
How many times can a killer shark be killed. 4 times apparently, definitely didn't need sequels.
The Matrix
The first movie stood on it's own two feet, the sequels were rubbish.
Shrek
Shrek 2 made sense, but the 3rd one was just cashing in.
Pirates of the Caribbean
As earlier, 2nd one was rubbish, 3rd one bloated and overdone and the 4th was surprisingly good.
The Mummy
First film stood on it's own, should have been left at that.
Cars
Didn't need a sequel, but it's popularity led to the 2nd film. You can understand it to a point, but it didn't need a sequel.
Ice Age
One of the few films were I understand the sequels, and each film has become better than the last.
Rocky
Rocky 2 made sense. Apollo's pride meant he was never going to keep to his word over there being no rematch. But Rocky 3 and 4 were mistakes and Rocky 5 was trying to cash in, but was dire.
My personal list of failed sequels include:
Ghostbusters 2 - why oh why did they make that film?
Transporter 2 and 3 - whats the point?
Smoking Aces 2 - about as much fun as chewing a cushion.
Beverly Hills Copy 3 - the 2nd one was hilarious, but to make a 3rd? It spoiled the first 2 because it was so rubbish.
Boon Dock Saints 2 - complete and total rubbish, made for the sake of a few fans who couldn't just accept that the first was a stand alone film.
Rambo 3 - what a mistake that turned out to be!
I could go on for ages, but you get the idea.
Good sequels though:
Bourne Identity - made sense to follow it through to the end.
Kill Bill - but essentially the 2 films were just one long story.
Lord of The Rings - no explanation needed
Again, could go on forever. I love films I do.