Minecraft, because there's pretty much no anti-cheat system. Even then, griefing is still incredibly easy to do, and most ways to limit griefing cause overly restrictive servers (which no one likes to play).
Otherwise, I can't think of anything. I would say Battlefield 3 because Punkbuster is incredibly easy to get around, and because EA-DICE refuses to even mention the game's name anymore, they aren't going to fix it. But even then, most hackers are once every thirty-some hours and usually get shut down through votekick pretty easily.
The7Sins said:
Team Fortress 2 before free to play was my favorite multiplayer game of all time and a serious contender for favorite game ever.
I still have not forgiven Valve for ruining a great game. Fuck you Valve. Fuck you.
Aren't you just the cutest little thing?