Zelda: The Legend of Zelda
FF: FFX or FFTA (depending on whether spinoffs count)
TES: Morrowind
Hearts of Iron: HoI 2
Civilization: Civ IV
Sonic: Sonic 2
Mario: Super Mario Bros 3 or Super Mario RPG (depending on whether the RPG series counts)
Pokemon: Gold/Silver, but X/Y is a close second
Starcraft: Brood War
SimCity: SimCity 4
Fallout: Either 1 or New Vegas depending on mood
Deus Ex: Original
Europa Universalis: IV (although III was also fantastic)
Assassin's Creed: II or IV (tied)
Age of Empires: II
Command & Conquer: Red Alert 2
Harvest Moon: Friends of Mineral Town
Torchlight: II
Warcraft: II but damn, III is good
Diablo: II but damn, III is good
Pac-Man: Pac-Man Championship Edition DX+
Banjo: Banjo-Kazooie
Yoshi: Yoshi's Island
Super Smash Bros: The original
Mario Kart: Mario Kart 64
Donkey Kong Country: DKC 2
Kirby: Kirby Super Star
World of Warcraft (expacs): Burning Crusade

I'm sure I'm leaving out some major series I've played... might come back and add more later