World of Warcraft.
I'm just getting into end-game stuff now. With this game Blizzard have created a seamless world where you can do anything you like. When you step back and look at the game they have made, you never cease to be amazed. And it only gets better the more you play. I currently play 6 characters - hunter, priest, druid, mage, warlock and warrior, and every class feels like playing a different game.
Aside from WOW, I play a bit of Battlefield 2 and a lot of serious flight simulation.
The greatest delight is to mark one's enemy, prepare everything, avenge oneself thoroughly, and then go to sleep.