This is such a hard question to answer. I have had a few staple games through my PC gaming experience. One of my all time favorites is Counter Strike. It was the first game I truly fell into a community experience. The game is STILL played today. So many mods and different types of servers. It led to me meeting so many good gaming friends. I remember late nights staying up and playing scoutzknivez flying around and sniping. I definitely look back on it fondly.
I soon became tired of CS and moved on to World of Warcraft. That game owned my soul for literally 7 years at least. I have finally kicked that habit, but the stories I have from the game make the 7 years worth it. Community is the most important part of of a game sometimes. With Warcraft, the people you play with make the game 10 times better.