When I applied to work at GAME in December '97, at the interview the then manager asked me what my favourite game was, to which I replied 'probably Pac Land on the PC Engine or Unirally on the SNES'. He just looked at me and said 'but they're years old'. Remember kids, always think before applying for a job in a shop where the manager is 5 years younger than you...
I guess the oldest game I play regularly (when bored and no other game is available) would be Solitaire, introduced in Windows 3.0 (1990). That game hasn't changed much since Windows 3.0 and is on every Windows and Windows CE/Mobile I encountered since then...
My loyalties will always be to pre CD games Mario 3, Sonic, Mega Man, My Aforementioned Pac Man and Galaga. It's not just the game play but these games provide me and flood me with memories no modern game can give me. Getting my first console Christmas of 1987 and what console was that.... A SEGA FUCKING MASTER SYSTEM I no longer have it I tossed it a few years ago (after I deemed it worthless in resale) But the memories My dad was GOD at gangster down. I'll never forget getting mega man 2 when I was 8 and being locked outside my house waiting to play it. Or the excitement of saving up over an entire summer to buy an SNES The First time I played Street Fighter II in an Arcade (which is now a GNC) The HOURS of playing Genesis games with my cousin where he's be a dick and make me play as tails. My Cousin and I going through the first half of Xenogears shortly after my Grandmother died Countless more 16 and 8 bit memories were made once I started using Emulators. Beating some games using a keyboard seems like a huge feat to me now.
Pac-Man probably. I also have a few personal faves on the Atari 2600 that I play regularly. Wizard of Wor and Berzerk come to mind.