Voodoo 2 hands down. Not only did Voodoo 2 SLI remain a viable choice for 3d gaming for a long time, but it remained the dominant choice for far longer than the 9700, 9800, Geforce 6xxx cards, GeForce 4xxx cards, or anything else.
Both 3dfx and Nvidia released several cards after the Voodoo 2 hit the scene but failed to produce anything that outperformed Voodoo 2 until much later . . . for 3dfx, it took the Voodoo 3 to outperform Voodoo 2 SLI (and even then, it took the better Voodoo 3 cards to accomplish that). For Nvidia, some of the TNT cards outperformed the Voodoo 2 SLI setups, as I recall, though not by all that much. In all fairness, we are talking about two standalone 3d adapters . . . moving from standalone 3d cards to integrated 2d/3d cards was a painful but necessary step for the industry.