Far Cry 3
Final Fantasy VII
Mass Effect (original)
Dune II: The Building of a Dynasty
Warcraft II (marginally ahead of III)
Grand Theft Auto V (close call between this one and San Andreas, though)
Age of Empires (original)
Civilization IV
X-COM: Enemy Unknown
Skyrim
Assassin's Creed II
Streets of Rage 2
Sonic the Hedgehog 2
Uncharted 2
Resident Evil (original)
Football Manager 2012
Wolfenstein: Enemy Territory
Tom Clancy's Splinter Cell: Conviction
Halo 3
Grandia
Batman: Arkham Asylum
Destruction Derby (original)
Wipeout HD
Resistance: Fall of Man
Duke Nukem (original)
Road Rash II

Like Formy, I've played a lot of games from series but not any others from that particular series, so didn't include those. I've also played a lot of series where all the games sort of melded into one for me because it was so long ago or something, and then there are games where there's sort of a series but they aren't official, but are all in the spirit of the same intention but with different names and no story/character ties between them, so I didn't include them either.