On the mac I'd have to say World of Warcraft. Any other game just pales in comparison to the exhilarating fun I've had in that game, the many friends I've made throughout the years and the basic quality of gameplay.
However due to time issues I quit my account last summer and haven't returned to it yet, although I probably will, should I ever have enough time on my hands to feel bored.
So as to not entirely turn my back on video games and stay in contact with several friends, I purchased an xbox360 not to long ago. Strangely enough, it's been the single player games I've enjoyed most on the box. Number one would have to be Fallout 3 with the expansions. The atmosphere is great and the gameplay is fun (shooting off peoples heads in slow motion VATS mode just never gets old).
The xbox multiplayer games I've enjoyed most have been Gears of War 2 and Burnout Paradise (the party mode is really great).