According to NPD Group list The Sims 2 Pets Expansion from Electronic Arts is on top.
The list shows the top 10 selling PC games for the week of November 12 - November 18, 2006.
1. The Sims 2 Pets - Electronic Arts
2. Medieval II: Total War - Sega
3. World Of Warcraft - Blizzard
4. Neverwinter Nights 2 - Atari
5. Flight Simulator X Deluxe - Microsoft
6. EverQuest II: Echoes Of Faydwer - SOE
7. Battlefield 2142 - Electronic Arts
8. Call of Duty 2 - Activision
9. The Sims 2 - Electronic Arts
10. The Sims 2 Happy Holiday Stuff - Electronic Arts