Gen 5: Gran Turismo 2, I played that game non-stop with my dad.
Gen 6: FFX, simply an amazing game, soundtrack, story, characters, etc....
Gen 7: Mass Effect 2, the story was incredible IMO.
Gen 8: Witcher 3, fantastic story, fantastic side missions that set the standard on what a side mission should be. I loved the gameplay and has a fantastic soundtrack (Persona 5 is a close second)