Either Crash Bandicoot, or Abes Oddesy. Yeah, they're not as old as SNES games and such, but I grew up during the ninties. I played with the PS1. Metal Gear Solid and Final Fantasy 7 were my parents. I did have a computer that could play DooM and Duke Nukem 3D and such, but I ended up with consoles before the massive PC game boom (Don't get me wrong, I'm not a console fanboy. I just don't care about the console vs PC argument.).
But I think the oldest game I play when I've got nothing to do and no way of using my old laptop or consoles, I play Pong on my phone.