I have actually two favorites. One was the Matrox Millenium, that card was super fast for its day. Even today it can hold its own (in 2d of course).
The second is the Voodoo2 (and voodoo1, basically the same but 2 had more mem and can SLI) This card pretty much set the standard. It was fast, looked pretty damn cool and needed no IRQs! I miss the 3d addon card. I hate having to get rid of a perfectly good card to stay on the cutting edge.