I'd say Mercenaries: Playground of Destruction has the most craptastical ending ever. After spearheading a tank assault, evading fire from North Korean troops, finding and rescuing the former president of Korea, disarming a missile launch, dismantling a nuclear warhead, taking down the tyrannical General Choi Song and saving the Earth from nuclear annihilation all in one mission, what's my reward? A news report telling me that the Russian Mafia has taken over North Korea and that everyone's life has become twice as miserable.
I'm not kidding.