Silent Hill 2. My review says why.
I decided to dig up that review and gave it a look. I think you were a tad bit too harsh man.

Anyhow, I can\'t say what my all-time most disappointing game is, but I can name a few recent ones.
Metal Gear Solid 2 - yup, it\'s true.
Luigi\'s Mansion
Waverace: Bluestorm
Sonic Adventure 2
Phantasy Star Online
Now, it all depends on what criteria these decisions are based on. Disappointment, as in the game wound up sucking? Or disappointing in the fact that the game wasn\'t what you expected?
I like both MGS2 + LM, they just weren\'t what I expected, and somewhat of a letdown. They are far from being bad games though.
Waverace is nothing like it should have been, a weak showcase for the GC. SA2 left me in utter disappointment with the lack of Sonic, and repetitive gameplay. And PSO, well. I won\'t get into that one.
(btw - I\'m sure I can think of tons more, I\'m just drawing a blank @ this moment.)
- dm -