Playstation One. The graphics of the N64 really turned me off of the system, for the most part, but not as much as that bloody awful gamepad.
Don't get me wrong, as I did enjoy some of the games on the system, like Golden Eye, Ocarina of Time, Bomberman 64, Mario 64, Mario Kart, and Mario Party.
I'll always love the Playstation, as the first game I bought for it was Final Fantasy VII, and that'll always be one of my all time favorite games. Plus Gran Turismo II. Me and a group of friend played that religiously one summer until we had first place in every race, had every car, and every car optimized. Then, of course, the Playstation had, and has, probably the best gamepad ever made for a console.