Well, the Intellivision came out less than a year after the Atari VCS, so the oldest I've played and own would be the 2600. I have two wood grain models, but as far as I know, only one of them works.
In my opinion, the 2600 doesn't have many games worth playing enough to go through the trouble of getting the damn thing to work. The last time I tried it didn't happen, and it took my uncle and I the better part of an hour to get it all together. I really would like to try the Intellivision, though. My grandpa picked it up with a game for five dollars at a garage sale.
The oldest console I have hooked up now would be a model 2 Genesis, though the FC Twin plays NES games flawlessly.