What's the single, most impactful GPU you've ever bought?

For me, its the Nvidia GeForce 6600 that I bought nearly 20 years ago.

It was the first upgrade I ever made, turning my Dell Dimension prebuilt into a gaming powerhouse! I have fond memories of playing 3D games like Half-Life and Dawn of War, on on a CRT, into the wee small hours.

And of course, it started me on the PC upgrade/building path that I still love.

What GPU has made the most impact on you?