Sniper Team 4 said:
Resistance 2 baby. Kill 10,000 people in ranked online matches. Never. AGAIN.
Second one was Call of Duty: World at War.
I'm surprised you didn't say "Never. AGAIN." for CODWAW. THAT GAME...
The game itself isn't that hard, it's when you dive into Veteran difficulty that things start to get insane. And in order to get every achievement, you MUST finish the entire game on Veteran. It is one of the most difficult game in the series on Veteran; let's just say, it earned it's nickname "Call of Duty: Grenades at War" extremely well. In addition, there are two achievements that REQUIRE you to get to max prestige (the first is prestige 1, the other is max), which would require more patience than the campaign, but if you've played the multilayer recently, it's hacked to hell! I've played several games where random people were invincible, or everyone had infinite clips, one where everyone had low gravity, one person was walking on the skybox noob-tubing everyone... it was bad.
OT: There was a point in life where I was dedicated to getting 100% achievements in every game I played, but that time has passed as my free time has evaporated. I've gotten quite a few, such as AC2, Skyrim, Modern Warfare 1, 2, and 3, Mass Effect 1, 2, and 3, Fallout 3 and NV, and I think a few others.