Hahahahaha.  I KNEW that was bound to happen to some people.  I saw the box for it at Best Buy and they didn't do a very good job making sure people knew it was an add-on. 

Halo reach was extremely disappointing, but still a great game. Killzone 2 was also disappointing, it was my first ps3 game and I felt as if I wasted money.

360: Halo Reach, Magna Carta 2 (my own fault for not realizing it was made in Korea and not Japan), Darksiders

PS3: FFXIII (hated it),

Wii: Super Paper Mario (a stain on Nintendo's good name), Mario Kart Wii (they ruined the turbo boost), Skyward Sword (5 years for THAT?!)

GameCube: StarFox Adventures, Star Fox Assault... basically, Nintendo has just run that series into the ground. No way to recover, it's been tarnished for almost ten years now.

Dragon Age II.
Not worthy of its name.

Most disappointing for me are the one's that I heard a lot of good things about but for me turned out to be style over substance.

So I would nominate-

Metal Gear Solid 4 (Think I really should have played the first 3 first)
Killzone 2- Couldn't stand the controls and controller input lag and really hated Rico.

How many of those thread do we already have? I think I already posted in 2 of them, at least XD.

Well, personally, there are 2 games I never want to play again and where I really consider them a waste of money.
The first one would be Tales of Symphonia - Dawn of the New World, the game I complained already about in the other threads. While most Tales of games are great and while I KNEW this is going to be worse than the prequel (at least when it comes to characters), I decided to still give it a chance.
What did I get? A game with the WORST possible characters EVER, not just in Tales of or the RPG genre, but of ALL time, a very disappointing gameplay and an uninteresting story... no, a story that is simply boring as HELL.
So in short, when it comes to the most important aspects of an RPG (characters, story and gameplay), this game was a disappointment in ALL of them. I mean, if I don't expect much and STILL get disappointed THAT much... Well, I don't think I need to say more about it...

And new on rank 2 on my personal "Fuckthisshit" game list:

The Legend of Zelda - Skyward Sword. When Nintendo released the artwork of Link and Fi (or Phai, how she's called in english, I think), I was pretty hyped. After the (for me) letdown called Wind Waker, I expected something like Ocarina of Time or Majoras Mask... But guess what: Skyward Sword's art direction got scrapped, and instead we got some hybrid graphics (Normally, I don't mind cel shading, but... Not in Zelda... Honestly...) and the game literally destroyed the few things I liked about Wind Waker. And the aspects I hated in Wind Waker were still there, e.g. the empty as hell overworld... if you can even call that an overworld. I mean, Wind Waker at least had islands like Port Monee (or what it's called in the english version) or Draconia, but Skyward Sword? The sky had... what? 2 minigame islands and 1 islands with a sidequest, the rest were empty ones where treasure chests appeared. The dungeons were the EXACT opposite of what I want in Zelda. There were few, they were small, uninteresting and most of the time you didn't even have to think, but just use your WM+... What I want(ed) are/were dungeons like the Fire or Spirit Temple from OoT. There were MANY other things I really disliked about Skyward Sword, but I don't even want to write anymore about it... xD

Prototype I mean i was expecting crazy sh!t not just sh!t

I would add Starfox Assault, that game's flying levels were sooooo good, and showed what the game could be.  The broken 3rd person shooter parts brought a AAA down to just above average.

Ultimate Spider-Man let me down, too. It featured a great story and gorgeous graphics but the game was broken. It was basically racing across town for no reason with too many instant deaths if you make a slight wrong turn or fighting frustrating bosses. I hated that game.

When I came into this thread, I pondered long and hard which was the game I ever owned that dissapointed me the most. After coming to the conclusion that I have quite a large number of games that dissapointed me in various degrees, I came up with a Top 10 worst dissapointments I ever had:

1st. Lufia: Curse of the Sinistrals (DS) - The biggest dissapointment on this list andtrully the one that deserves the number 1 spot. Lufia: Curse of the Sinistrals is a re-telling of the story of Lufia II: Rise of the Sinistrals, my favourite game of all time, so it was with some degree of expectation and fear that I awaited this game. The screens looked good as did the previews, so the expectations rose as the game was set to be released. What we got in the end was frankly an extremely generic and exhausted action RPG that had none of the Lufia series charm and a pretty extensive butchering of the story of Lufia II. I could frankly write a thesis on this, but this link sums up pretty nicely all the things that went wrong with this "re-telling": http://wwwthinkinginsidethebox.blogspot.pt/2011/02/lufia-curse-of-sinistrals.html. Having said that, the game did somethings quite nicely, like the Grid system, and revisiting some of the best places in Lufia II in upgraded graphics (Parcelyte town, Tanbel mines, etc) was gorgeous, but those are just some tiny specks of brilliance amidst a big pile of dung.

2nd. The 3rd Birthday (PSP) - I admit it, I used to defend this game before it released. Not because I was hoping it to be as good as the original Parasite Eve games, but because I wanted to give SE the benefit of the doubt, and how wrong was I. Even though SE did warn us that they were going to take the PE series from scratch and do it all over again, T3B is probably the worst way to do it. Not only do they deconstruct an amazing character such as Aya Brea, even though it's revealed in the end that it's not actually her (the damage had been already done), they introduce a story that is probably one of the worst that SE ever did. Having said that, and after detatching myself long enough from the story of the game, I found T3B's gameplay to be quite satisfactory, though the lack of a second analog stick on the PSP probably neutered a lot of the potential that the combat system could have had. 

3rd. Shadow Hearts: From a New World (PS2) - It's pretty hard to screw up a storyline that had been constructed from two previous games as bad as From a New World did. Without any kind of resemblance or throwback to the first two games except the combat system and some character cameos (which even had their own original story butchered), you felt like you weren't actually playing a Shadow Hearts game. Of the original dark, deep and sometimes scary as heck plot, we ended up getting a light hearted plot about an american boy investigating why a series of monsters pop out from strange "windows" here and there which wasn't that well explained at all and mixes it up with the actual evil being his thought-to-be dead sister. All in all, it was a mess of a plot and undeserving of the name Shadow Hearts.

4th. Grandia Xtreme (PS2) - A pretty Xtreme (pun intended) departure from the Grandia series, which had it's beginnings on Grandia III, Grandia Xtreme was a mixture of the worst aspects of Grandia and Grandia II with an extremely obnoxious grind-fest system that could annoy even the most stalwart and patient gamer. Throw in a forgettable story, shallow characters and nothing actually remarkable, Grandia Xtreme sadly dictated the end of the Grandia series, a series that had amazing potential and could have rivalled with the JRPG behemots.

5th. Chrono Cross (PS) - The sequel that was never meant to be a sequel. Even it's developers didn't intend for it to be a sequel, though as we all know, developers wishes rarely matter when it comes to marketing ploys and overall company desires. The game that was destined to be called Radical Dreamers, a story which added in parallel to the story of Chrono Trigger ended up being a poorly constructed sequel with so many contradictions you could fill an ocean with them. Not a bad game in itself, though far from being a masterpiece like Trigger was, the single fact of making us believe that all the events of Trigger didn't actually matter and that the emotional link we constructed with Chrono and the rest of the gang were basically fake, destroyed all the good that Chrono Cross could ever have.

6th. Escape from Monkey Island (PC) - Probably the weakest Monkey Island game, it was still a very good game. My dissapointment with it stems more from the fact that it came right after the amazing Curse of the Monkey Island, one of my favourite games of all time, one of the funniest games I ever played and probably one of the funniest things in any kind of medium I ever seen. Sadly, Escape didn't add anything of the sort to the series and even managed to extensively dumb down one of the hallmarks of the series, the insult swordfights. Adding to that, the addition of the Monkey Kombat was crap at best.

7th. Far Cry 2 (PS3) - From all the games on this list, this is the only one I didn't ever buy myself, rather it was a birthday gift. Having enjoyed the original Far Cry a lot, my expectations for FC2 were rather high as well. Sadly the game didn't really deserve the Far Cry name mostly because of how broken the sandbox aspects of the game really are and how tedious and boring it is. There's nothing fun in having to constantly fight off unstoppable waves of enemies when all you want to do is explore the map. You can't go 5 minutes in the game without someone shooting at you, even if you're in areas that are supposed to be devoid of any kind of enemy. I ended up trading it (one of the very few games I ever did).

8th. Final Fantasy III (NES) - Probably the most generic Final Fantasy game ever made, both in story aspects and characters. There's basically no remarkable feature that you can point to in those departments. While the remakes have done the game a much more favourable job, the original FFIII is still regarded as one of the worst FF games ever made. Having said that, FFIII laid the roots for the Job system that future Final Fantasy games would use, like FFV and FFXI. 

9th. Age of Empires III (PC) - This was mostly a dissapointment because it lacked all the charm that the previous 2 AoE games had. By going for a much more realistic approach (but ending up somewhere in between, as there was a rather extensive cartoonification in the game, notably on units), AoE III ended up being too similar to pretty much every other RTS game in the market at the time. Also, one of the best things about AoE games, the MP, had a large amount of problems, especially in the LAN department (which in the 2 previous games was the paramount MP choice) and even the SP campaing was lackluster, with horrible voice acting to add up to the issues. 

10th. Equinox (SNES) - Probably one of the worst game I ever played, it was talked to and hyped to exhaustion both in EGM and Superjuegos back in 93/94. One of the many Sony published games on a Nintendo system, it was by far the worst one. Back then, as a little kid, I saved up for a shit ton of time to get this game only to not having the mental fortitude or capacity to play it for more than 5 minutes (even nowadays), mostly on how bad it is.