I say the original NES that came out in the 80's. Nothing beats Bart vs. the Space Mutants or Blades of Steel, except for all the other classics like Tecmo Superbowl and Duckhunt
Well, I'm going to exclude PC's from my thoughts, because no console can really have the staying power of a PC, especially considering that if you wait long enough, a PC will be able to emulate a console anyway...
I think it's a dead heat between the SNES and PS1 in my mind... The SNES really broke quite a bit of ground, including the first truly great polygonal game (Starfox) But it wasn't really until the PS1 that designers were able to realize their visions of the future... Sure new concepts will arise, and there will be ground breaking new game types as technology increases, but everything 3d will be rooted to the PS2.