MW2 Campaign on Veteran. You can get killed in less than a second if even one enemy is shooting at you. Checkpoints always seem so far away after you die 50 times on the same checkpoint that you just got. However, some levels are much easier. Also, there is no unlimited respawning of the enemies. When you kill somebody, they are down for good. But, there's a million enemies anyway, so you have to camp most of the time, and if you are not paying attention for one second, you're back to the last checkpoint.
