The best co-op games of all time

What are the best co-op games? Games offer up magic in lots of different guises, but working with a friend to achieve a goal is, arguably, where they shine. So what are the titles that will pull friends together and offer the best cooperative group-based thrills?The great thing about the best PC games involving co-op play is that you're not restricted to a genre. As long as there's room for two controllers on a split screen or some perfectly coordinated play over a headset, co-op games can be f...