My favorites:
Voodoo, because it was the first product that changed my way of thinking about PC gaming
Voodoo 5500, because it was the first product, for me, that offered an ability to improve the quality of the game, with their high quality RGSSSAA.
ATI Radeon 9700 Pro, because it offered a nice balance of performance/AA/AF quality with an impressive feature set.
ATI Radeon 1900XTX, because if offered a nice balance of performance/AA/AF quality with an impressive feature set.