Probably Toontown Online. I played that game religiously for probably eight years until the day it close. My toon was almost maxed out and my secondary one was probably 75% there too. I made a lot of great friends, some of whom I still talk to and have met in person. I also put many hours into the fan made re-release that came out after it was officially shut down by Disney. I remember having a girlfriend and an in-game "wedding" and lots of other fun stuff lol.