My favourite online shooter is Left4Dead2. I've had so much fun playing the game for hours on end. I remember it being the first online game I had on Steam. It had Co-op as well as versus modes, out of which I especially like the Versus as it gave me the actual gameplay for a being a zombie. Pulling players down railings, charging them down the roofs of skyscrapers, punching them in the face as a tank, it was insanely addictive. Custom maps, modes and awesome playerbase made it really enjoyable. I do wish they would make a third one already.