Of all the games I played, Animal Crossing was the only one to grip me with such a ritualistic obsession that I would, for a time, get up early before school just to check on my town at 6 am, and come home and play it for hours on end. This period didn't last for more than a couple months, but when it had me, it had me.

In terms of raw play time, nothing could compete with Super Smash Bros. Melee or Halo 2, though I bet Halo 3 and Super Smash Bros. Brawl will eventually surpass them.