The oldest systems I have that still work are my original Game Boy and N64. I usually only break out the N64 for the occasional Goldeneye or Mario Kart session with my friends, or better yet, some awesome Pokemon Puzzle League action.
I had an SNES when I was a kid, but I (very foolishly) gave it away. Then a friend of mine gave me one for my birthday a couple years ago, but sadly it stopped working after only about two months, right after I bought Mario RPG on Ebay. Money well spent...