Civilization 4 with Beyond the Sword Expansion (It will get you addicted)
Company of Heroes with Opposing Fronts Expansion (Amazing experience...)
Portal (Just try them out.. hehe)
Half-Life 2 with Episode 1 and 2 Expansion (You'll be surprised how real a game can be)
*Buy the Orange Box for above 2 games
World of Warcraft (Still the most balanced and perfect MMORPG out there... But mind ya, this thing's like crack)
Mabinogi (Another MMORPG that gives a strong niche due to its unique fighting system. This one's a lesser of a crack than WoW but still requires you to play alot)
I wouldn't recommend Diablo 2 because the graphic is too outdated and battle.net service has some lag issues. (I doubt they're still maintaining the server after 8 freaking years..) Also you will get frustrated from the endless boss-huntings for items and runes... Try Neverwinter Nights 2 for better graphics with D&D twist.
Bioshock and Crysis for one-time awesome experience. (They are the games with best graphic out there so far)
All very good games.
I think nowdays console games dominate the gaming world (GTA4, MGS4, etc). Maybe things will change once Starcraft 2 and Diablo 3 come out... :) (Hail Blizzard!)