Charlie

World of Warcraft - perhaps the most obvious choice for me, this game provides a huge wealth of player interaction. One of my primary interests in design is intercultural interaction, but also basically how people communicate with each other (the masses vs. the minority). There's player-player interaction, player-computer interaction, the often elaborate system guilds make to fight big bosses and distribute loot, in-game and out-of-game communication, etc.

Other games I'd consider are The Sims (largely due to it's huge fanbase of customization, and the psychology behind why people are so into the game) and Second Life (in a way, a combination of the Sims and the World of Warcraft).