The most disappointing would probably be The Witcher. I didn't get very far in, so I'm not going to doubt you if you tell me the story was fantastic or that it improved dramatically, but I can only judge based on what I played, and from that, I hated the combat. I thought it was clunky, awkward, and unintuitive. I also thought the game was incredibly ugly, and I'm not usually one to care about "graphics". I suppose the main reason I was so disappointed was that I was prepared for and expecting gaming brilliance from the get-go, mostly based on what others have said.

Absolute worst game I've ever played? I'm gonna go with the E.T. game for the Atari 2600. One of the most confusing experiences of my life. It made absolutely zero sense. It was like a scientific experiment into unfun. And I tried to like it, I really did, because you just didn't have access to that many games back then. I played it many times and it always ended in frustration and annoyance. It was a vile, vile thing.

If I must elaborate...

With Banjo Tooie Rare went waaaaaay overboard with their collectathon stylings.  The worlds were far too large, and several were difficult to navigate.  Glitter Glutch Mine, Cuckoo Cloud Land, Terrydactylland and Grunty Industries were just far too inaccessible for a platforming game.  This is all complicated by the fact that you essentially have to navigate levels with five different characters (Banjo-Kazooie, solo Banjo, solo Kazooie, Mumbo, and Wumba's transformation) which means a lot of backtracking.  It just doesn't have the pick up and play appeal of Banjo 1 or Super Mario 64.

Tactics 2 idk.  I was just bored to tears by the story, and didn't find myself caring about anything that was going on.

Nintendo of America made the right call by releasing Doki Doki Panic over here, because Super Mario 2 Japan was pretty bad.  There are good challenging games, and then there are ridiculous ones.  Springs that launch you too high to actually see yourself or plan your landings are a major pain, half of the game involves jumping from single block in the middle of nowhere to jumping to the next single block in the middle of nowhere.  Your choice of character is either Luigi who controls like he's wearing butter shoes or Mario who can't jump high enough for most of the game's challenges.

Worst of all is the fact that the game punishes you for exploration.  In the original Mario, you were tempted to hit every block and check every warp pipe to see what hidden things await you.  This time, if you go too far off the beaten path... bam.  Backwards warp zone.  That'll teach you to try and find secrets you little bastard.

Oh and to access certain hidden levels YOU HAVE TO BEAT THE GAME 8 TIMES! FOR REAL?!

I think a lot of people defend this game out of some misplaced national pride.  "Oh, they think this game was too hard for us Americans?  Hah, not for me.  I'm super hardcore man."  But in reality, it's an awful game with awful level design.  Some games (like Battletoads or Mega Man) are challenging but rewarding. Mario 2 is just plain frustrating.  If this game would have come out in the US, it would have ruined a lot of Christmas mornings.

It opens up a bit at certain parts, but it always returns to the scripted/linear structure.

Most disappointing- Devil May Cry 4, Dark Souls, Mass Effect 2

For all these games, the main reason I was disappointed was that I went in with really high expectations. Demon's Souls and Devil May Cry 3 are both in my top 5 favorite games of all time. In the case of Devil May Cry 4 and Dark Souls, I still consider them to be good games. They just come nowhere near the level of their predecessors.

For Mass Effect 2, I remember Mass Effect 1 fondly, but my reaction to Mass Effect 2 makes me wonder if my taste in games has changed because I consider it to be an above-average game at best. I was so disappointed by it that I have yet to play Mass Effect 3 and have no plans to play it in the future.

As a Dragon Ball Z fan who just bought a PS3  "Ultimate tenkaichi" was the worst choice for  a first impressions of PS3 capabilities.The combat mechanics ruined the game for me I can easily say that "Budokai Tenkaichi" series on PS2 are extremely more enjoyable than "Ultimate Tenkaichi" .

Well it seems some have been putting out the most dissapointing games.
The Most Dissapointing game for me is DK64, because there are five characters that you play as, th you have to collect 500 bananas to complete the game 101% and the fact that those crappy minigames exists like the gnawty one and another one that I do not remember. The power ups sucked too.

call of duty 3 (wii) / no words to describe how bad this game is. And its the only game I regret buying this gen

The most disappointing is easily Final Fantasy XIII.

There was nothing to it besides a series of battles followed by cut scenes. All sense of adventure and discovering the world in the previous games was lost. This was partly because of the extremely linear levels but also because they decided that everything you did find would be shoved into huge encyclopaedia. Some of the worst design decisions made in a game. Having said that, I'm not sure it can count as the worst game I've ever played as there was a period of fun I had in the middle of thinking "When's this game going to get good?" and "This game is horrible".

Worst game however, that probably goes to Call of Duty 3 (Wii).

Poor level design, invisible walls, massive lag and uninspired gameplay. It tainted all following Treyarch titles for me. After playing the previous two Call of Duty games on PC, this was horrible. Even the set-pieces that were well executed in CoD2 were either absent or downright horrible. The controls weren't anything special either.

Honourable mentions: Tomb Raider Angel of Darkness, Duke Nukem Forever, Legacy of Kain: Blood Omen 2, Far Cry 2.

resident evil 5 was disapointing in my opinion. I"m not big on co-op and did'nt like being forced to play with an A.I. partner and the enviroments and situations were just boring and lame.

I love FFXI, it's my favorite MMO ever, and I wanted to love FFXIV so bad.  I remember arguing with people before it launched that the quests limits were a good thing and how the job changing would revolutionize the way we play MMO's but now I am just so jaded.  A Realm Reborn could be great but I'm pretty bitter about the whole thing so it's going to have to be amazing to make me want to come back.

Just thinking back to the completely unfinished bug-riddled game, the mess of a UI, the terrible limits, and the complete lack of attention is just starting to make me angry again.  Even the things that made XIV cool, the job changing on a whim and the way it makes you part of the story, has already been done much better on other MMO's.  They're really going to have to step up their game to make an already old feeling MMO feel new.

But anyways, yeah, I forgot what server my old FFXI linkshell all joined or even if they still play FFXIV so I can start new if it ends up being really good.