As far as largest failure of an anticipated game goes I'd say Sonic next gen hands down. I may not be a Sonic fan, but its hard to argue against it. When Sega announced it fans were going crazy and were eager to get a new 3D Sonic game on the new consoles. What they got was an unfinished and untested game that was full of rehashes. It is arguably the largest example of how to disappoint a fanbase. Where other games can be called bad, or even unfinished in some cases at least you aren't going through poorly made stages 4 times and getting stuck in invisible walls in the middle of a map.
For me in specific the greatest failure would be Crash of the Titans. I was a die-hard Crash fan (after all it was my first game) all good in the PS games and where the PS2 games started to drop the ball at least they had the same principles, mechanics, and atmosphere of their predecessors. Being play as a crazy brainless marsupial, break stuff by spinning, and beat up a foolish scientist (or his former pets in Twinsanity) while having plenty of humor in Twinsanity's case. Then with Crash of the Titans they mess with the characters, try to make a simple series into a more childish story, and worst of all they destroyed the game-play. You like spinning well now your main attack is punching and spinning is a more limited move. They also focus more on the controlling the enemies instead of controlling Crash. The most terrifing thing is that Crash (a once completely silent hero due to his stupidity) speaks. It is a complete abomination to any fan who loved Crash from the early days. It makes me sad that the game I started with has devolved into such a state and the worst part is that this has happened to many great games though they have mixed success.