World of Warcraft for sure. My brother and I had downloaded the trial as a joke between us, making fun of the nerds... Next thing you know we could not log off. The size, the community, the music was just so captivating; I can still hear the score in my heard when I think of Teldrassil. The friends I made in game became closer than anyone I knew prior and some I still know and see today! It was lightning in a bottle for me and once my enthusiasm for WoW finally drained years later, no other MMO could really replace it.