As a first generation gamer, I have to disagree with some of these. Here is my top 5 list:
World of Warcraft-Love it or hate it, it's the biggest money maker out there. I tried to hate it and did so for years but recently broke down and installed it from some discs laying around the house. I'm addicted. I try hard NOT to play this game.
Oblivion-Such a great game. Whether you want to do a real quest or wander around enjoying scenery, this game is terrific. I've logged over 400 hours without even starting the main quest once. Maybe I will one day.
Fallout 2- This is the best turn based strategy/rpg game ever.
The Sims 2-The amount of options in this life sim are staggering. Personally, I prefer logging into my friends/relatives games and creating communities of hot lesbians, wookies, or tv casts to surprise them upon their return.
System Shock 2-FPS/RPG hybrid that is probably the only game that will always make me jump out of my chair. Great atmosphere and very hard.