I don't think I can break it down into just one game, it's that hard, would have to go by console and even that's tough.
SNES - Mario Kart or Theme Park World (Cant separate them, and feel bad for leaving out Super Mario World...)
PS1 - CTR: Crash Team Racing (Was going to go with something like Rayman or Croc, but CTR was a beast of a game)
Gameboy Color/SP - Pokemon Yellow, Sapphire and Fire Red (These 3 were super fun and I loved playing them on the handhelds)
Gamecube - Ace Golf (This console was in my opinion the most underrated console ever, so many good games)
PS2 - Brian Lara International Cricket/ Pro Evolution Soccer (Best 2 Sports games ever made, shoutout to Ratchet and Clank!)
Xbox 360 - Call of Duty: Black Ops/SSX (Best gaming period of my life, and the most hated remaster which I loved.)
PS3 - LittleBigPlanet (Probably the best of what I played)
PS4 - Legend of Heroes: Trails of Cold Steel 1 & 2 (Cant separate em, and began my love for JRPGs. Shoutout to Horizon as well, which made me interested in single-player games again)
Mac Gaming - Minecraft/ Puzzle Pirates (Mac gaming sucked, but Minecraft was amazing, and Puzzle Pirates was the bomb!)
PC Gaming - I've played some fantastic games that are so different but so great. RDR2, Rimworld, Tales of Arise, No Mans Sky, Trails of Cold Steel 3... I can't separate any of them.