Saw this thread over at the WoW Forums so I thought I'd bring it here where the selection might be a bit more diverse.
10. Shadowrun (Sega Genesis)
09. The Legend of Zelda (NES)
08. Baseball Stars (NES)
07. Diablo (PC)
06. Police Quest (PC)
05. Castlevania 2: Simon's Quest (NES)
04. The Legend of Zelda: The Windwaker (Gamecube)
03. River City Ransom (NES)
02. King's Quest 1-4 (PC)
01. World of Warcraft (PC)
I know I'm forgetting a lot of others that probably could have been on the list, but these are the ones that occur to me at the moment.