OK, worst game ever is covered, best game ever, best game with monkeys in ball covered...
So how about this... what game was the single biggest let down you ever expirenced?
I've got a couple... Zoda's Revenge was bad but honestly I wasn't expecting it to live up to the original Star Tropics...
And I didn't really enjoy the multiplayer frame-rate of Mario Kart 64.
The runner up (for me anyway) is Ocarina of Time for the N64/Gamecube. A lot of people call this the best video game ever made, but I just found the entire quest to be terminally boring. The empty overworld with little or no badguys between each level was so very dull that I quit playing five times before I beat it (as in stopped playing it for months or years), and I found the dungeons to be tedious and the puzzles to be more annoying then enjoyable. Unlike the temples in Wind Waker or even Majora's Mask.
Speaking of which, Ocarina of Time was so boring that I bought Majora's Mask the day it came out and I didn't even muster up the courage to open the thing and actually play it until over a year later.
But the game that was the biggest let down overall for me was Donkey Kong 64. First off, I really, really enjoyed the first two Donkey Kong Country games on the SNES (thought #3 sucked ass), and I had enjoyed Banjo-Kazooie enough to be psyched for DK64. Instead, I found a game that crawled along at a snail's pace. And the game relied too much on collecting crap then on fun levels and objectives. It could have been fun but lots of the non-DK characters were just no fun to use.
So yep, DK64 is the biggest letdown I ever experienced... but your milage may very.