King's Bounty : The Legend : I'm a huge fan of HoMM series and that one looked promising. Nice graphics (if you like the style, which i do), but that's pretty much all. Once you've started, you're in for hours and hours of tedious fights with monsters. Basically, think of it as a HoMM where they would just have kept the fights : no castle to develop, no mines to capture. Okay, the "wife" system is new, but it could have been so much better... As it is, it's just a pretext to expanded stacks of inventory and bonuses...

Ditto Oblivion. I actually think it's a great game, it's just a massive disappointment next to Morrowind.
Ditto Yoshi's Story, too. Abominably bad game. Short, too. I think it only took me 45 minutes or so to beat it. Yoshi's Island was good, too, so I don't know how they made the sequel so awful.
I have to disagree on RE4 and 5. I can 100% see why series fans would say that, as they're not survival horror games anymore and the gameplay's changed radically, but I absolutely love both of them, probably the best action games ever made - I'm playing through 5 now. I feel like I'm playing a Hollywood blockbuster.
Silent Hill 5 was something I was pleasantly surprised by. I thought it was good, especially after SH4 - which was dreadful. I liked Origins, too.
For me, the big one was FF12. Kind of like the Oblivion, RE4/5 critiques, it just wasn't a FF game. Not a bad game, but it felt like an MMO and lacked the life-destroying addictiveness of other FF games. Also, it was a lot like Star Wars.
In terms of PC games - Mistmare. What a turd that was. As a theology and history buff, it looked awesome. Then I played it. Ugggghhh. Worst game ever.

Silent Hill 4 is a game you either 100% love or hate. I happen to absolutely love it, and think it's possibly the scariest in the series. Whereas 2 was psychological horror, 4 seemed to play on Japanese ghost stories, and theres just something creepy in that it took place in an apartment, like, say, the one I live in.
I'm actually playing through 4 right now, and aside from the limited inventory system, I'm thoroughly enjoying it.

Actually, I mostly encounter SH4 as a game SH series fans either hate or think is OK. For Japanese ghost stories, I'd go with Fatal Frame. Great series.
It just wasn't a SH game, even though the story was good. Plus it had some major problems. Are you more than halfway through the game?
If not, you might want to read this when you're done. I marked for spoilers. Minor spoilers regarding gameplay, not plot, but just in case, I marked.
*minor gameplay spoilers*
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Around the halfway point, you will literally backtrack through half of the game in a an escort mission. I thought the game was mediocre before that, and just terrible after. I don't know anyone who found that portion enjoyable. There's also some other changes that make the game substantially more annoying around there.

BLACK & WHITE
- Definately the most disapointing game ever.

Blood Bowl on the 360
Not because its a bad game really but because the lack of support for anything but single games over xbox live utterly destroyed the primary objective that my mates and I got it for, to recreate our old tabletop tournaments over the net.
The theory was that we'd all buy a copy and play whenever we had time without having to drag stuff around to someone else's house at a time when everyone was free. Of course there ARE tournaments in the game but they're local multiplayer only meaning we STILL have to drag stuff around to other people's houses, just that instead of miniatures its memory cards and controllers.
Of course, with miniatures I'd have saved $80 on buying the game (which is no fun against the cheating prick of an AI) and have been able to use my Undead & Halfling teams that aren't in the computer version...

Dreamkiller. This game is not a game it is just like a nightmare.

I've beaten all the Silent Hills before, I'm just replaying them all in a row. The Eileen portions can be frustrating, but it is nice being able to just run ahead and she magically appears in whatever door you go into.
And I've never played the others, but Fatal Frame is insane. I mean that in a good way, but I have jumped out of my chair numerous times playing it.
Another good horror game is Siren Blood Curse for PS3. There are some frustrating portions, but the story is genuinely creepy. I have the PS2 version but the Japanese people speaking with British accents is a bit too much.

Wait what? Everytime I ran ahead, she got left behind in the previous room. Those escort segments absolutely ruined the game in my opinion. If they completely took out the frustrating babysitting, i'd probably like it. >_<

Spore = ones of the worst purchases of my life

Since you asked for 3:
Deus Ex Invisible War
Gothic
Beyond Divinity
I played each for at least 10 hours before uninstalling due to boredom.

The most disappointing game that I can remember is Giants to be honest. I was expecting to shit my pants with awesomeness and I just cried myself to sleep. Then I installed Tex Murphy Under a Killing Moon thinking that it would also suck and I should've never bought a game on GOG but fell in love with it. Fuck you Giants: Citizen Kabuko!

How was I never informed about this game?! Having a health boob sounds awesome.

I never liked C&C 3.
Maybe because it felt like previous games recycled with better graphics, maybe not

Halo: What? This is supposed to be a GOOD game? Generic at best.
Far Cry: Very pretty, but as much personality as a corpse. Enemy AI was brilliant to begin with, then the developers just stopped trying. I hate you CryTek.
Command and Conquer: Loved it to bits, can't replay it without an immense feeling of frustration, which is a shame. Those FMV sequences are awesome.

IMO C&C3 is still 1000 times better than RA3.