I would have to say Pac-Man and Asteroids on my old Light Sixer. I loved that game too! And it was probably the only reason why I still have a volume from the Namco Museum for PSone.
I play Pong every few years. I play Asteroids and Galaga whenever I see them on an arcade machine, usually a few times a year. I play Digger (1983) once a month, I used to play it more when I carried my DS or PSP with me but now my only portable game system is my phone and it doesn't play well with touchscreens. I ported Digger to Wii in 2012. I play GTA2 (1999) multiplayer almost every day. I used to play Worms Armageddon (1999?) multiplayer a lot but now I get my turned based artillery action from Hedgewars.
I play Ponx on the Atari Lynx, which is basically the original Pong game ported to the Lynx, so I guess that counts as the oldest game that I still play, if you simply consider Ponx to be Pong.
Demon Attack still gets a lot of play in my house, as well as No Escape and Beamrider, all 3 being on the 2600.
Super Mario World or The Legend of Zelda: A Link to the Past . Been playing those quite a lot at the moment.
Recently? Been playing the shit out of Super Mario Bros. 'cause I just picked up a NES. That'll wear off though. I also play Streets of Rage on MD fairly regularly. Very regularly though is Command and Conquer Red Alert Retaliation on PS1. All the damn time. I used to play through Crash Bandicoot 2 at least once a year, too, until last year when I 100%'d it with all the gems. That one might be laid to rest now. OH! Tetris for Game Boy. It's the only game boy game I make sure I've always got a copy of, the rest come and go on eBay.