Hmmm ... probably the PC. That goes back to 1982 doesn't it?
Though via it I have a choice of old Windows stuff, natively (~1990 - yes, I don't need a 386 or virtualPC to run that stuff, if you do, something's gone wrong), old DOS stuff either natively or in DOSbox (right back to 1983 if I REALLY get a hankering for Alley Cat or Arcade Volleyball ... which I really, really don't), and emulators for various things, particularly Atari ST, SMS/Genesis/SNES or C64/Spectrum.
Mostly though it's the PC and ST stuff. The Atari in my cupboard has a ROM burn date of Nov. 20th 1985 on its desktop info screen (heh, I'll have to do something for it's 25th birthday - damn near forgot!), so that's surely going back far enough.
Prefer to do that nowadays rather than firing up the original equipment, which is a bulky pain in the ass to set up and is quite happily mouldering in the cupboard (and the RGB monitors under cover in the attic) until I have sufficient space to devote to it.
Actual hardware that I have and would still play if said space/time - and in some cases, repair skills - were available:
Atari STF 1mb (1985)
Sega Master System II (198?)
Sinclair Spectrum 128k +2 (1986 or so for the system, 1982 for the platform)
If we tot it up over the past few years of actual play on actual hardware however, the most number of hours will probably have gone into, in order:
PC (2006 system, 1982(DOS)/1995(Win32)/2001(XP) platform, games from late 80s thru to present)
Gameboy Pocket (late 90s system, 1989 platform)
Playstation ('94?)
PS2 (i forget)
Wii, belonging to friends
Sega Gamegear (1990)
Bro's XBox360 and PS3 at random points
Random game 'n' watch type things (various ages)
Arcades. Including a classic Bomberman, AND a coffeetable Galaga ('82?)
Though I've put in a good few mac hours, I never found a single game to play on them
