Top 3 disappointing games ever for me:
1) Deus Ex Invisible War - Deus Ex was always going to be a difficult game to follow on from (especially with the multiple endings available), but the plot wasn't great and the engine was terrible, made worse by having to fit into the XBox's available memory. This also damaged Thief 3 (made with the same engine, but more optimised by then), but at least that had a fantastic story so you could try and forget the tiny levels.
2) Global Power - advertised as a global politics simulator, this had lofty goals and failed to meet any of them. You could change all sorts of stuff in your chosen country, but none of it made any difference. After a few years of game play, most people ended up invading neighbouring countries simply to have something happen, to be greeted with ascii-art combat that you had no control over.
3) Empire Total War - graphics were improved from previous instalments, but lots of units were made the same so the factions didn't feel particularly different to each other. Definitely a step back from Medieval 2 and Rome (which was the peak of the series in my opinion).

Rise Of The Robots - Hyped to death, not the worst beat em up of all time, but not far off. Cool music by Brian May though.
GTA4 - unfinished and uninteresting (the expansions run a lot better. Why no patch for the original though?). Some people liked this, i didn't, sorry.
Doom 3 - The engine was amazing, the begining and ending are fantastic, it's the 4 hours in between that sucked.

There you go. Quests and dialogues aren't working thou :D

One 1 Uno: Gothic 3
Retail was the worst game in history. melee was completely broken an an unusable skill. most animals could stun lock you and you would sit there and die, most enemy's would stunlock you and you would die, and the world was a glitchy shutter filled mess with loading all over the place. ((community patch and work with the devs made a decent game in the end)) oh and some of the skills were useless or didn't work.
Two 2 Dos: Spacesiege
This was absolute crap, no support, bugs everywhere, no patches, no way to dodge. everything is mouse driven and its short and annoying. Shadowgrounds did it much better.
Three 3 Tres: Monster Madness
Wont even play on Nvidia cards, bad rendering issues. Patches actually break the game further. glitchy bug filled and even on easy the boss enemy's chew up up and spit you out. a wonderful concept, best played on console, never for the PC.
The Bards Tale -- funny and so very sad at the same time
FFXII -- best FF game i ever played and actually beat, nothing came close, this is how they all should have been imo
Assassin's Creed -- Own 2 versions of it and loved it to death, so much fun to just explore and get lost in.
Fallout 3 -- Own 2 versions of it and love it to death, still playing it since release.
Oblivion -- Own 3 versions of it and love it to death, played the hell out of it for years
S.T.A.L.K.E.R. -- Bad ass open world, took a little patching but its a damn good game now.
I enjoyed the above but after reading all the sadness and lost possibility with Mrorrwind and Oblivion im going to see if i can find a copy of Daggerfall and see what this wondrous world truly is.

Gothic 3
Forgot about that one. The community patch still doesn't help on my system unfortunately, so i still wasted �30 on a pile of crap, not the developers fault though, they just didn't have time to finish it. Thankfully they have just released Risen which plays brilliantly and Gothic fans will get straight into it. (PC version that is, the xbox360 version is terrible).

Sooo, since there were only 3... you liked 1, and possibly 2? So you mean 3, right? Why didn't you just say that? Or are you counting expansion packs as different games? If so, there's a pretty good reason why those are "more of the same" (they're supposed to be).
And you will note that while I really didn't like 3 (it was in my list), you have to admit it did have entirely new ideas, like the cards, and the supply stations, as well as the natives.
I have entirely no idea what you're talking about here - ALL RTS games require you to build stuff before the other guy. Maybe you didn't like it - which is fair - but I don't get your logic at all.

Hellgate London - Very bugged. Online multiplayer is dead. Game itself is boring. I spend 50$ on it. There's not much more to say about this game.
The Movies - The tycoon aspect of it can be fun, for a few hours only. What was most dissapointg was the main aspect of it, the aspect that made me buy this game: the creation of movies. It's very very limited, you just select pre made scenes and pre made camera angles and pow, it's done. Sure, you can add your own music and record your voice in game, but that doesn't save it.
Rayman 3 - Not a bad game, but being a huge fan of Rayman 2, I expected much, much more. The gameplay was worse, the characters and the levels weren't as nice as R2, and the story was ridiculous.
You can download Daggerfall for free on the Oblivion's website: http://www.elderscrolls.com/downloads/downloads_games.htm

I hope you enjoy it, although I think it might be quite jarring . I started with Daggerfall and then played Morrowind. I later tired Arena and found it hard to adjust back to that old interface. I might have to give it a try again sometime.
Daggerfall and Arena

Ninja Gaiden II.
My God, they took every single thing that made the original Xbox game my favourite Action game ever, and systematically pissed all over it. Enemy and weapon balance? Gone. Good level design? None. Bosses? Boring.
Funniest thing about it was that NG was always really fair even on the hardest setting. NGII is just ridiculously cheap with off screen attacks and instant deaths on higher settings. And yet it's still much, much easier on all settings than the equivalents in the original.

Might & Magic IX was horrible. They took everything that was good about Might & Magic and threw it away. Instead, they added a worse (!) graphics engine and lots of bugs.
Simon the Sorcerer 3D, from what I read.
Super Mario Bros. 2 (USA). They stole the game, made new graphics and released it as a Mario game, instead of publishing the real Super Mario Bros. 2 (well, they later did as "Lost Levels"). The game still was ok, but not that idea. It got even worse with the numbering of Final Fantasy on SNES, though. But at least there they labeled it worse than it was and still had real FF games...
And the special prize goes to....
Duke Nukem Forever
After over a decade, they have gone insolvent. So chances are it will never appear. If this isn't disappointing...

3 of my most disappointing games ever:
Resident Evil 5; Even though RE4 already had moved away from the classic RE-style, it still was a great game and it was the best game on the Gamecube. So I had big hopes for Resident Evil 5. Really BIG hopes. I read every bit of information I could find. watched all the trailers and still had my hopes up. When the release came closer and closer I was acting more and more like a schoolgirl outside a Backstreet Boys concert (just like every other games that I'm looking forward to) and then the game came. And what a shitload of diarrhoea vomit that game was. I'm talking about the singelplayer aspect of the game here. The co-op part of the game was a blast and RE5 are one of the best co-op games out there. But as a Resident Evil and a singel-player game? Then it really blows. If I can even call it a Resident Evil game. Next time Capcom, don't lay all your resources on the co-op. Please make the singel-player part, you know, playable. At least they are gonna go back to the roots with Resident Evil 6. I'm gonna be happy if the zombies return, and not that shit we see in Left 4 Dead 'cause thoose are not zombies.
Ninja Gaiden 2: What do you get by pissing on everything that was great in the first game?
Half-life 2: The first game was great and I still love it (Can't wait for Black Mesa) but the sequel was a big disappointment. It was boring, predictable and repetitive.The episodes wasn't that much better. Can't wait to play Episode 3, which will probably come out in 2012.

Don't you have anything better to do at christmas than look down with spite on those who does not share your mainstream views?

I don't rate HL2 much either. It was exactly what you said it was. Too many repetitive sections where you do the same thing over and over until you move onto the next - like the Vehicle parts where you drive through a tunnel, stop, get out of the vehicle, kill a few people, get back in, drive through next tunnel. Do this about 6 times until you reach some boss. Or the stupid section with the ant-lions that drags on and on. They pop up out of the sand if you walk on the sand, but stick a bit of cardboard down and walk on that and they don't care -- utter nonsense. The game feels like a series of mini-games at times, as opposed to a standard shooter; the end part with the super grav gun where you HAVE to use it, thus taking away all sense of freedom rounded off for me, a massively overrated game. I simply loved HL 1, it's a non-stop rollercoaster ride, until the poor Xen part which I mostly ignore and pretend isn't part of the game, but to me, HL2 is as disappointing to play as the aforementioned Xen section of its predecessor.

Oddly, I really liked Far Cry 2. I liked it more than the original, at least.
Morrowind - I just found it... Bleh. The combat did my tree in, I got lost, never had any idea what I was doing... You get the idea.
Dungeons & Dragons: Dragonshard - Was really hyped up about it, installed it, played it for 5 mins, turned it off (was going to do something else IIRC) and it's never booted since.
Two Worlds - I was really looking forward to sinking my teeth into it. But... Poor voice acting, next to no Character Customisation, poor controls and it's just a mess. So glad I didn't buy it at full price.