Perhaps safe for Super Smash Bros. Melee, I've never played a game quite so much as Phantasy Star Online.
It was great - it did great things.
I mean, given, Super Smash Bros. Melee was an excellent game. Not counting a few hiccups, the balancing was pretty solid (Unless you were a pro, then it's FOX FOX FOX), and the gameplay was revolutionary. The advent of pressing a single button to use a special technique was utterly mystifying after I was so illusioned by Mortal Kombat and Street Fighter - and it works flawlessly. The game is utterly accessible, with absolutely no daunting factors. Everyone can jump in at any time - it's a rare experience where one can play the same game on VS. for hours, all the while knowing there's no reward in-game - it's just a ton of fun. Really hearkens back to the days of good competitive multiplayer for multplayers sake.