I wanna say Bioshock 2, but if anything, Bioshock 2 was a good game, just no where near as good as Bioshock 1 and it's only looked down upon because it let a lot of folks down.
That said, I wanna go with Modern Warfare 2. I enjoyed the original Modern Warfare campaign, I thought it was fun, enjoyable, challenging Veteran, good story and ideas. Modern Warfare 2 however just threw all that way and said "LET'S PUT IN MORE GUNS AND MAKE THEM GO BANG BANG!". It's story was really lackluster and the veteran is just a piss poor excuse for a difficulty.