The Hidden: Source. Most likely the game I play best.
Players are usually trigger-happy, paranoid or a mixture of both. Not hard to make players shoot teammates, just jump in-between them and then jump away. Grenades, stabs, taunts, jumping in front of them and then away again. Every action just serves to feed their paranoid thoughts.
I remember a few years back a mate of mine had his own server, he added a few things extra, like handicaps (Turning the Hidden into a beacon, knife damage reduced, health reduce. etc.) removing the pigstick (A attack with the knife that took about 3 seconds to charge up. It was unlimited use with no cooldown) and replacing it with a "pigpush", which pushes targets a few feet away instead of instantly killing them. Good days. Shame no one in Australia or New Zealand plays anymore.