Ok. I'm going to judge this based on the game with which I have clocked the most hours ever.
Samurai Warriors 2.
I know, I know, but it's true. I often still jump into bed and play a level of this before going to sleep of a night. The simple, meaningless carnage is somehow soothing.
But, looking objectively at the games I've played over the years (and still play now and then on emulaters)
Atari: Pitfall & River Raid. My preteens, right there.
Sega - Sonic series, Shining Force series, Strike Series, Road Rash 2, Micro Machines 2 (remember the J-Cart? Madness!), Pitfall: The Mayan Adventure (simultaneously nostalgic and technically impressive at the time of it's release), Flashback - too many others to mention. Megadrive was THE system that made me a gamer.
Playstation - Tekken 2&3, Resident Evil, Max Payne, GTA Series, Infamous
Nintendo - Chrono Trigger, GoldenEye/Perfect Dark, LOZ: OoT, Mario Kart, Eternal Darkness; Sanity's Requim
X-Box - Fable, Halo, Bioshock, Alan Wake, Batman Arkham Asylum.
PC - Doom Series, Dune: The Battle for Arrakis, Dues Ex, Fallout Series (although NV was a letdown) Jagged Alliance 2, Baldur's Gate series, Half-Life Series
Oh, and though I don't play it anymore it has to be mentioned for the 3 years I played it and made many good friends doing so : World of Warcraft.
God damn, and I was trying to be succint.....