When I discovered emulation, I started playing some real old stuff again. I've got Atari games like Frogger. That's probably the oldest game I've played again most recently, but I don't play that regularly.

I intermittently fire up Micromachines 2 (Mega Drive), which is just as much fun as it has always been.

If we're taking about the oldest game I still play regularly and seriously (that I did start playing back in the day) and still play now, that would be some PSP and PS2 games. I'm currently playing Wipeout Pulse and Gran Turismo 4 and progressing (even got the same save files from the PSP). I also regularly play Tony Hawk's Underground, Lumines, Grand Theft Auto China Town Wars and Everybody's Golf..... But these have fallen out of the current game rotation.

Overall if we're going to crown an undisputible winner, I think that will be the original Tetris.

Edit: also reminds me of Sim City 3000 and X-wing Alliance, which I will remember and play again once a decade. I still mourn the loss of the playerbase of Quake 3 Arena and Quake Live, otherwise I would still happily play those now.