Gaming research firm NPD Group revealed May’s top 20 selling PC games and unsurprisingly World of Warcraft: Wrath of the Lich King sits at number one. The original title was the best-selling PC game of 2005 and 2006, and the Burning Crusade expansion sold 2.4 million copies in its first 24 hours. With more people playing WoW than ever, Wrath of the Lich King sold 2.8 million copies on its opening day, making it the fastest selling PC game of all time.

While WoW’s second expansion has been resting comfortably at the top of NPD’s list since its debut, the World of Warcraft Battle Chest and other standalone titles never fall far behind. Last month the Battle Chest was at number 3, World of Warcraft at 4, and the Burning Crusade at 11. Other Blizzard titles to make this month’s list include the StarCraft Battle Chest at 10, the Diablo Battle Chest at 16 and the Warcraft III Battle Chest at 18.

Coming in second place is Electronic Arts' The Sims 2 Double Deluxe. EA holds other positions with Spore at 6, The Sims 2 Apartment Life at 8 and The Sim City Box at 14. Left 4 Dead is hanging tight at position 7 and Fallout 3 moved up three spots from last month to claim spot 15. Despite Demigod’s rampant piracy and Stardock’s reported technical issues, it came in at number 12.

NPD Group research excludes both legal and illegal digital distribution, so its numbers are not completely accurate as they mostly reflect what's hot on retail shelves. The complete top 20 list, after the jump.

1. World of Warcraft: Wrath of the Lich King - Blizzard
2. The Sims 2 Double Deluxe - Electronic Arts
3. World of Warcraft Battle Chest - Blizzard
4. World of Warcraft - Blizzard
5. Empire: Total War - Creative Assembly/Sega
6. Spore - Electronic Arts
7. Left 4 Dead - Valve
8. The Sims 2 Apartment Life - Electronic Arts
9. Fallout Trilogy - Interplay
10. StarCraft: Battle Chest - Blizzard
11. World of Warcraft: Burning Crusade - Blizzard
12. Demigod - Gas Powered Games/Stardock
13. Real Deal Slots Adventure - Phantom EFX
14. The SimCity Box - Electronic Arts
15. Fallout 3 - Bethesda Softworks
16. Diablo Battle Chest - Blizzard
17. Elder Scrolls IV: Oblivion Game of the Year Edition - Bethesda Softworks
18. Warcraft III Battle Chest - Blizzard
19. Warhammer 40K: Dawn of War II - Relic/THQ
20. Company of Heroes: Tales of Valor - Relic/THQ