One game that comes to mind is saints row 3. It was so watered down compared to saints row 2

Unreal 2

Not a bad fps by any means (a solid 75%), but nowhere near the excellence of the first game. Some major gripes include: overabundance of enemies, deeply dissatisfying endgame and weird roleplay elements. Seems like the game could've been a whole lot more. I wouldn't be surprised to find that development was rushed.

All of those are fine choices.

Invisible War was particularly disappointing given how good Deus Ex was/is.
I don't know how they managed to make a game where the design was inferior in every way to it's predecessor. Ugh; those horribly small maps as well. All they had to do was clone the first game to make lots of $$$$

The weird 2D/3D mix in Commandos 3 didn't float my boat; Commandos 2 still looks gorgeous. Didn't like the timed mission aspect either.

I can't say I was particularly shocked by Bethesda's take on "Fallout" given how Oblivion turned out.

I could probably add Curse of Monkey Island to the list although Escape from Monkey Island is meant to be even worse. I don't consider those to be real sequels without Ron Gilbert at the helm.

XCOM APOC is a horribly balanced mess of turn based + real time combat with terrible battlescape character models and ufopedia art. XCOM vs Barney the dinosaur + headcrabs.
There's actually some good stuff buried underneath all the garbage - mainly the cityscape.

I wasn't that impressed by Thief 3 (Deadly Shadows) either - I did like Robbing the Cradle, that left an impression.

I hated Metal Gear Solid 2 - bought the clunky PC release and tried playing using keyboard controls!
Nothing like the first Metal Gear Solid (i.e. PSX) game at all.
I wanted to be Snake not Raiden - didn't read a review; just assumed it would be directly following on from the first.
All the monotonous dialog and wacky story/characters. Trolling? Or meant to be an insult?
Splinter Cell did just what I was looking for fortunately.

Wow.

I too much prefer the original Witcher, but did you find the second to be such a disappointment because you found the first to be so great or did you find a lot of things with it you didn't like?

As for my list:

Diablo 3
Prince of Persia: The Warrior Within
Final Fantasy VII, VIII, X
Burnout Paradise
LEGO Star Wars Clone wars
Borderlands 2 before Gaige was introduced.

Oblivion easily takes that dubious honor. Remove everything I liked about Morrowind, and you get Oblivion.

Oh, man I'd forgotten about this title. Not only is it a horrible sequel its a horrible end to an iconic RPG series.

I'm going to name Deus Ex: Invisible War as another bust. The sad thing about it is that if they dropped the Deus Ex name it would have had a far better reception.

A game I haven't seen name yet is Unreal Tournament 3. The graphics were such an over valued and poorly implemented part of that game that it was released it a horrid state. Once it was patched it ended up being more of the same but much less of it.

Ditto and Ditto.

Loved TS.

My vote goes to Driver 3 (PS2). I loved Driver and Driver 2, I bought this game blind based on that... letting Tanner get out of the car was a HUGE mistake, it was very poorly done and made the game feel like a cheap GTA3 knockoff and almost unplayable. I struggled through it, but I learned my lesson at that time to NEVER buy a game blindly on release. So it wasn't a total waste.

~

Funny, the save system was my favourite feature of that game!
I only played this series recently, but I definitely agree that 3 was much worse than the others. I enjoyed Kyrandia 1 and especially Kyrandia 2, but I couldn't finish Kyrandia 3.

Now you're trying to pick a fight!

...anyways... Came to add the disappointment that is Blood 2.

Resident Evil 6... Game's alright in co-op if you're dicking around with friends. But as a horror game it fails even worse than RE5 did.

Yes, Monkey Island 3 was a let-down after the two brilliant predecessors. But with contenders like Ultima 9 I wouldn't call MI3 the 'Most Disappointing Seqel Ever'.

Diablo 3,Unrel 2,Fallout 3,Quake 4,all Alien games after AvP 2 and so on...wow thats a lot of dissapointment

Blood II

Diablo II
TES Oblivion
Deus Ex: Invisible War
Fallout 3