This is a silly question. Why? Because people are automatically listing an old card as the "worst" video card they ever owned. Naturally, cards from way back when will look pale in comparison to today's cards. It is like asking the question: what is the worst CPU you ever owned? My answer would be the CPU used in the Tandy Color Computer from way back when.
To make their answers valid, they should compare their cards to what was available at the time they purchased that worst card.
For me, the Neon 250 is the worst card I ever owned. Poor drivers. Not very fast. The works. At the time I bought the Neon 250, the following cards were available (i.e. were released to the market at MOST one year before): G400/MAX, TNT2 Ultras, Voodoo3s, GeForce256, Rage 128GL/Pro, Rage MAXX, and the Savage2000.
I owned the Rage 128GL, G400MAX, 2 TNT2 Ultras, Voodoo3 2000, and GeForce256 SDR at the time I bought the Neon 250. So I know that the Neon 250 stunk in comparison. It works so it was not terribly pathetic. I'm just saying as a comparison to the others, it is not very good. Therefore, it was the worst card I ever owned.