No online game will ever beat the hours I spent playing Baldur's Gate or Doom. And the level of enjoyment I had is yet to be matched. (well, not entirely true... Jet Set Willy on Spectrum is number one).
The type of enjoyment I received from online games is mostly resumed to having to put up with a lot of jerks and narrow-minded brats who enjoy ruining your fun in benefit of their own fun, stuckup admins/coders/gods/whatevers with delusions of grandeur. Some enjoyment is also possible, of course. A few individuals do know how to behave and "play the game" and there is always the fact that the game itself was meant to be fun (very few games fail on design). But at the end of the day I would have a false notion of enjoyment that I quickly recognized and made me stop playing those kind of games. And I wasn't even paying.
