The first ones that come to mind are:

Good Sequels

• Uncharted 2 - Did what every sequel should and dumped everything that didn't work in the first game (like sixaxis motion control stuff) and improved on the game in every other department.
  
• Timesplitters 2 - Took an awesome game and made it even more awesome by stuffing in a ton of fun and amusing content.
  
• Splinter Cell Chaos Theory - Best game of the series without question. The perfect stealth game.
  
• Battlefield: Bad Company 2 - Improved on the first game in every way, and produced one of the best multiplayer experiences of this generation in the process.
  
• Street Fighter IV - It brought the classic 2D plane fighter back in style.

Bad Sequels

• Splinter Cell: Double Agent - I just don't know what they were thinking when they made this game.
  
• Dawn of War 2 - Has a decent campaign for a single run, but doesn't have the endless replayability of the first game.

This?

http://t2.gstatic.com/images?q=tbn:ANd9GcRsS9DeLzXdxNA1CJ4mph4Hm6s-Waus0hjbTBCKQ_Xcf4V5Cfp11A

Blasphemy.

Very good Uncharted 2

Very very bad Fable 2

Oh yes, yes indeed. Curse still relied on the insanity of its own world. Once the series got to metahumour, it just didn't feel right anymore. That's also why, despite being somewhat charming on their own, the Tales episodes just can't hold a candle, or even a dimly flickering match, to the first three.

What?

What?

Surely you refer to Mario is Missing or Mario's Time Machine or perhaps Hotel Mario

Worst - Boondock Saints 2: All Saints Day

Best - Red Dragon

Or games...

Worst - Final Fantasy X-2

Best - God of War 3

The game category was much harder than the movie one...

These, although I'd like to also add that I didn't enjoy DoW 2's campaign either. Getting rid of the base building was a unbelievably daft decision.

And, I'm surprised I'm the first to mention that on my bad sequel list is Oblivion.

Now, let me make this perfectly clear, I love Oblivion. Enjoyed the hell out of it. It just lost the magic Morrowind had. Maybe it's because I'm older, but I could have "nothing to do" in Morrowind and enjoy myself, I couldn't do that in Oblivion. Which is why, even now, 9 years on, I still play Morrowind.

lets see,

Fable two is way better than fable one, if for nothing else the dog and guns
RE4 is so much better than the ones before it, the fifth one wasn't as good imo, but it wasn't bad either
I enjoyed ob far more than morrowind, but i played ob first
Pokemon gold improved on everything from the first games, probably my favorite game from when i was a kid
Red dead redemption, altho its more like reboot from the first game


Worst....gee i can't really think of any games that the sequel ruined forever
DA2 isn't terrible, i mean i didn't think DAO was that great to begin with, but it was rushed, and could use some polish...or more cave designs
RE5 just wasn't as good as RE4, but it was still alot of fun playing with my cousin
Crackdown 2 seemed kinda lazy and used the exact map from the first one almost, i didn't buy it but i played the demo long enough

Also worst movie sequel ever was boon dock saints 2, seemed to lose the feel of the first one

Best movie sequel was the dark knight, just cause of the joker

a lot of hate for fable 2 going on

the first fable i was like meh, it was such generalised fantasy,
the second, halfway through i was like meh, and then... i had kids...
completely and totally won over, i changed from an evil character to a good, completely melted, became one of my favourite games because it made me feel like that, hats off for that achievement alone, surely,
but it really raised the bar as far as voice acting in games goes, the gta series is the only thing that compares in that regard

This. Sadly though, you can also name Heroes of Might and Magic IV as one of the worst sequels, and V wasn't much better. Hopefully, VI can bring the series back to it's roots, but I don't exactly have high hopes.

this. amen brother

Best: Not really sure, not many sequels are better than the first.

Worst: Fable 2 by far

Why? If I may ask? It was the best game in the series for me.

Best: Halo 3: ODST, Dead Space 2, Crysis 2, Killzone 2/3, Assassin's Creed II

Worst: All Call of Duty titles.

Best (by far): Thief: The Metal Age
Also good: Dark Forces II: Jedi Knight, Rome: Total War

Worst: KoToR II
Also Bad: Any C&C after Zero Hour, HL2 was not as good as it should have been, Call of Duty after 2,

Worst sequal: Mass Effect 2

Best Sequal: Lots of them

Since people are weighing in on the movie front:

Best:

Terminator 2
Star Trek - The Wrath of Khan (as relative to its predecessor, though it was Star Trek VI that was the pinnacle)
Casino Royale

Worst: Well, other than MOST of them, I specifically (currently) recall my extreme disappointment in the following:

Teenage Mutant Ninja Turtles 3
X-men: The Last Stand
Spider-man 3
Indiana Jones and the Crystal Skull

...All of which killed (or should have killed) their franchises, if only temporarily.

:blink: Surely you jest. The best James Bond sequel was From Russia With Love. I don't want to start a Craig/Connery debate, but I personally would put Casino Royale in my 'worst sequels' category.

Aw, that was my favorite one. Time traveling NInja Turtle Samurai!

There have been many excellent sequels to games. The best ones I can think of:

Age of Empires 2
Diablo 2
Hitman 2
Half-Life 2
Doom 2
Ratchet & Clank 2
Soulcalibur 2
Red Alert 2
SimCity 2000
MechAssault 2
KotOR 2 (yeah, I loved it)

And that's just the twos. I could go on.

Worst sequels? There's a few of them too.

Age of Empires 3
Modern Warfare 2
Dungeon Keeper 2
Empire Earth 3
Any Final Fantasy after VI

I do not. James Bond had become a joke, with ever-more implausible villians meeting their doom thanks to ever-more-implausible scenes and gadgets, all while Brosnan refused to break a sweat. It became a farce, because the series as a whole had long since jumped the shark. Now, the argument can be made that this was, in part, the whole point of the series, but it was clearly losing its grip among the fan base, and a return to a grittier, (slightly) more realistic Bond was needed, and Casino Royale delivered with impressive results. For me, it had nothing to do with Craig - any Bond lead man would have done the trick, so long as they were willing to get their hands dirty. I enjoyed a good many of the other sequels, regardless of their levels of camp - Goldeneye was one of my favorites, actually. But the reboot was absolutely needed.