90% of games are best with a mouse and keyboard. The exceptions are platformers (both 3D and side-scrolling; also includes generic 3rd-person 3D action games), racing games, and fighting games.
Most games that originally came out on an older console (SNES games or older) are best with a controller as well...
but new games, 99% of the time, are best with a mouse and keyboard. Assuming you have a 2-botton scroll wheel mouse, you have a controller with 111 keys (108 + 3) and pretty much instant turning speed. Sound much better than an Xbox controller to me (*shudders at sound of Xbox*).