Specific games, not series.
And it can't be a game you played like a week, month or even a year ago, has to be like games that you find timeless and have/would have played several times throughout several parts of your life.
For me it would be.
Front Mission 3
Final Fantasy 9
Diablo 2
World of Warcraft (Since vanilla)
Unreal Tournament 2004
Hitman Blood Money
Hitman 2 Silent Assassin
Fallout Tactics
Fallout 2
Icewind Dale 2
Battlefield 2
Star Wars Battlefront 2
Civilizations 3
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Star Wars Battlefront 1 & 2 Borderlands 1 & 2 Fallout 3 Need For Speed Most Wanted (original) Assassins Creed 2 Pokemon Heart Gold Skate 2 MX vs ATV Unleashed