Most disappointing of all time: Master of Orion 3
I loved the first two, and I waited so long for the third, but it turned out to be pretty mediocre.
Most disappointing recently: Left 4 Dead 2
I never played the first one, and all the hype had me expecting a great game. Instead I found it to be quite annoying and not very fun. I guess I've been spoiled by modern FPS game engines that have more realistic feeling weapons, destructible objects in the environment, the ability to "zoom aim" with all guns, and don't feel like you're speed skating everywhere you move.