I've been gaming since the Atari 2600, and this is actually a pretty easy question for me.
Everquest. Up until the Planes of Power expansion, it is the best game ever made, imo. Nothing else has ever pulled me in like that one did. I moved on to WoW for a while, but it never captured the feel of that first time in EQ. I played EQ since beta, and my Ranger is like the 4th oldest character on one of the original servers.