What is the most disappointing sequel you have ever played?

Like the title says, what is the most disappointing sequel you have ever played.
For me it is Bioshock 2, not that it was bad, but it just didn't have what made the first game so great. The story was ok and the setting was in the same old Rapture. Sure the gameplay worked better and the A.I. was better, but there weren't any big plot twists or scares or great environments that the first had and the characters weren't as crazy and maniacal. The multiplayer was lackluster, glitchy, and laggy (Wasn't expecting much, but still could have been more polished.) and got boring after getting to about level 20. 

With the first being my favorite game I've ever played, the second just didn't feel right and could have had some more time to be more polished than it was.

Fallout 3.

Assassin's Creed II. And, I can't really even tell you why. I was all excited for it after loving the first one, sat down and played it for a while, then returned it to GameFly. It was just no fun at all. It made me start to think that the first one was just a fluke, and if I played it now I would hate that one, too.

I remember Devil May Cry 2 being a pretty big disappointment back in the day.

Devil May Cry 2 and Mass Effect 2 are the only things that immediately come to mind.

" Assassin's Creed II. And, I can't really even tell you why. I was all excited for it after loving the first one, sat down and played it for a while, then returned it to GameFly. It was just no fun at all. It made me start to think that the first one was just a fluke, and if I played it now I would hate that one, too. "

I'll go with Gears 2, Fable 2, and Splinter Cell Conviction if that counts.

Deus Ex: Invisible War and Command & Conquer: Tiberian Sun. Without a doubt those two are games where you literally feel your soul crying because of the huge letdown.

Black and White 2
I loved the first so much, but the second felt watered down and simplistic.

The most recent Bionic Commando 

" @AjayRaz: I only played the Wolfenstein demo but can say that I loved Return to Castle Wolfenstein's multiplayer "

Devil May Cry 2, easliy.

Final Fantasy X-2.

All the Command and Conquer and red alert sequels.  The units got weirder and weirder.  Tiberian Sun was already too dumb for me.   Just give me normal tanks, apcs, artillery, jets and subs.

" Devil May Cry 2, easliy. "

I'll also go with Star Wars: The Old Republic mmo. It's not even out yet, but it still disappoints.

Deus Ex: Invisible War don't like the protagonist or the story, JC Denton's dialogue and voice to me was what drew me into the story. At first I thought he was voiced by Kevin Conroy

Metal Gear Solid 4. Easy choice, that game was... not amazing.

ut3

Just Cause 2

Modern Warfare 2 (PC)

After the demo, Crackdown 2.

Zelda 2.

Halo 2, I hated that game. Prince of Persia: Warrior Within was also a bit of a bummer, though it still was a pretty good game.
 
I would add Gunstar Super Heroes, but that's more of a remake than sequel.

Streets of Rage 3. Double Dragon 3.
 
3 is a cursed number I guess.. wait a sec... Gears of War... 3?
 
Fuck!

Halo 3/reach beta. 
But since i can just custom gametype shit so that people move faster/jump higher/ no damn armorlock not really reach. Although the clip sizes sucked.

Most recently, Command & Conquer 4.

Star Wars: Knights of the Old Republic 2: The Sith Lords 
 
I'm one of the few people that didn't like how combat played in KOTOR, I wanted a more action oriented mechanic. The attack-queue mechanic I felt just made the whole game too slow. I bought the sequel hoping they had changed it, but it was exactly the same so I didn't play it. Yeah, I'm a bastard.

I never got around to finishing Banjo Tooie after really enjoying the first one 5 years ago. It suffered the overwhelming content ambition Rare stumbled upon by 2000. I also never finished DK64 because of that.

Resistance 2. God the first game was so fun, and was really the first game i got serious into playing online. Insomniac cheated themselves out of a good game when they thought it'd be a good idea by taking everything every other shooter was doing and cramming it in their game.

Super Mario 2.

Any of the Spyro games that were made after the Psone games. That series has completely lost its charm after Insomniac left it in the hands of Sierra. Whatever happened to home worlds that had portals that would split off to colourful imaginative lands and you had a set number of things to collect in the land to 100% complete it? Spyro doesn't even look the same anymore, a mere shadow of his former self.

I'd probably say Oblivion.
 
I can't really think of another sequel that totally disappointed me, but Oblivion was a let down. I still played it a lot, but it lost so much of what made Morrowind one of my favorite games. It tried to be an action RPG too much, in my opinion, with poor results. I mean, Morrowind's action was horrible in terms of action games, but it wasn't about that, so it was alright. Oblivion tried to make it about that too much, and ended up missing the point to me.

Off the top of my head, I'm gonna say Skate 2. It freaking sucks. That series hasn't progressed an inch since the original. EA has been way too lazy with the series. The first one was good and had plenty of potential for growth, but they still haven't made the games any better. At all.

need for speed carbon  
and for some reason modern warfare2 
 
i have always liked cod 4, but as soon as i got mw2 i realised that the two games were very different. thats why I, like many others have gone back to cod 4

Assassin's Creed: Bloodlines, if you can even call it a sequel. Really horrible game.
Also, Mario Kart 64. Super Mario Kart was awesome, but MK64 was boring.
Spider-Man 3 also comes to mind.

Rise of Legends hurt pretty bad, especially because it diminished Big Huge Games greatly. They had a wonderful start with Rise of Nations, it seemed like they were going to be a competing name in the real time strategy genre. Not so much....
 
TES IV: Oblivion gets a special mention though. The presentation went up and the depth of the game world came careening down. It's still a great game, but heartbreaking if you played previous TES.

" Assassin's Creed II. And, I can't really even tell you why. I was all excited for it after loving the first one, sat down and played it for a while, then returned it to GameFly. It was just no fun at all. It made me start to think that the first one was just a fluke, and if I played it now I would hate that one, too. "

Fable 2 
Call Of Duty 3

All Final Fantasy games after 8 
All Tony Hawk games after American Wasteland  
Super Mario 2
 Pokemon, All of them

Fallout 3, (and probably new vegas... we'll see) uhh, MGS3,  every MK game since MK3U,  invisible war (I loved the first deus ex)    thats all i can think of now...  I'll edit this later.
 
EDIT: Yup told ya i'd forget something...   MW2 was okay, but I didn't really like the last few parts of the game...  resident evil 5 was okay too but I wanted something more - maybe a return to the old style

Broken Sword: The Angel of Death.  

KoTOR 2 just because if it was actually finished it could have been so fucking amazing.