So people don't have to read every single post to get an idea, here are the greatest system-selling franchises according to GAF in order. Results taken from the first three pages (150 posts).
Metal Gear Solid
Mentioned in 35 posts
Super Mario
Mentioned in 32 posts
Final Fantasy
Mentioned in 26 posts
The Legend of Zelda
Mentioned in 24 posts
Halo
Mentioned in 15 posts
Resident Evil
Mentioned in 12 posts
Super Smash Bros.
Mentioned in 11 posts
Soul Edge/Calibur, Grand Theft Auto, Metroid & Pokémon
All mentioned in 8 posts
Gran Turismo, Gears of War & Dead Rising(!)
All mentioned in 7 posts
Street Fighter
Mentioned in 6 posts
God of War, Sonic the Hedgehog & Blu-ray :lol
All mentioned in 5 posts
Castlevania
Mentioned in 4 posts
Mario Kart, Ninja Gaiden, Tekken, Phantasy Star, Shenmue & Mass Effect
Mentioned in 3 posts
Ratchet & Clank, Disgaea & everything by Team ICO
All mentioned twice
Bar none the most mentioned individual game is Metal Gear Solid 4. Most surprising to me are the amount of Resident Evil fans in this topic, the way Dead Rising actually sold 360's for some, and how Soul Calibur is up there with friggin GTA and Pokémon. |: