If I had to list the most overhyped games of all time (sequels, naturally) they would be in such an order:
1. GTA IV
2. Halo 3
3. MGS IV
However I can't use these, since I had partially given up hope for a mircale after the previous installment to all three. I had hope, but don't we all.
For me personally it would have to be Assassins Creed; looked great, atmosphere great, but nothing to do and you can't even run without the game hating you. Devil May Cry 2, and yes I almost did cry- also applies to the second half of DMC4- it was simply no fun at all to play and the storyline felt like it was stolen out of a fortune cookie or budget christmas cracker. Really liked DMC3, sadly let down by the predecessor and successor alike.
Resident Evil 5 to an extent, it was fun and all, and I had some epic multiplayer moments ("Hey mate, watch what my shotgun does to the licker tank") but once you've cleared it a single time there's really no motivation to go through all that again.
Every Sonic game since Adventure 2 (Battle). You know why.
Final Fantasy X, not because I felt it a bad game, but after playing the previous three titles and finding them all terribly engrossing I just felt like ten was something to do in between the other games I was anticipating the release of. Traded it in, but would actually like to give it another go through some point in the near future.
And in light of that I would like to direct attention to one of the most hyped games of all time (second only to the dreadful GTAIV) : Final Fantasy VII!
The one game, I must say, that took my expectations and made them laughably inferior. Sometimes hype can be right on the money.