1. World of Warcraft

2. Warcraft III/Dota

3. Counter-Strike: Source

4. Team Fortress 2/Orange Box

5. Battlefield 2

1) Counter Strike

2) Warcraft 3

3) Age of Empires 2

4) World of Warcraft

5) Ultima Online

Just checked and saw you were in the US. This is the Amazon US link.

My top 5-

1 Half Life 2 and sequels
2 Baldurs Gate 2
3 Bioshock
4 Dragon Age
5 Medieval 2: Total War

1. System Shock 2
2. Civilization II
3. Max Payne 1 & 2
4. AD&D (All of them) Baldur's Gate, Icewind Dale, old Gold Box Series, Eye of the Beholder etc..
5. Mass Effect Series.

1. World of Warcraft - this damn frickin game...

2. Warcraft 3 - love this one.

3. AoE The Rise of Rome - Classic.

4. Bang! Gunship Elite - Really fun space shooter.

5. Guns - The duel - badass online TPS/RPG matrixy game.

half life 2
starcraft
diablo 2
battlefield 2
too many games to narrow down for a 5th spot lol

I will check them out after I am done with god of war III. I am saving money right now. So I cant be spending too much on games! 200 hours is crazy though.

@Sharky:
You are not alone in holding Starcraft as gaming bliss. But you asked for 5. There are too many great PC games:)
(It would have been my number 6:)

1. Heroes of Might & Magic 3
2. Civilization IV
3. Mass Effect 2
3. Fallout 3
4. Warhammer 40k: Dawn of War

You can easily spend 200 hours on BG2 alone if you take on most of the side quests.