Definitely Voodoo 1. Seeing the improvement it did to the visual quality of Tomb Raider over my Matrox Mystique was simply breathtaking. The magic was "texture filtering" (if I remember it correctly). Voodoo 2 was a cool card, but only faster than the Voodoo 1, however Voodoo 1 was a quantum leap.
Today I would say that the Radeon 9700 pro has the potential to become my new all-time champion. It is the first time, that you can play 32bit color, high-resolution, AA and AF with decent framerates for any existing game. It is the first time since Voodoo 1, that I have experienced more than an incremental improvement to 3D image quality (I am not talking about the signal quality here).
Cheers
Speedy