I would say the nvidia 6600GT and 7600GT for bringing awesome performance for price and SLI to the mainstream. I miss those days. Probably the radeon X1950PRO's as well for bringing the crossfire bridges instead of the PITA dongle!
On another note I agree with the Radeon X800 series. Image quality was amazing and the fact that you could unlock them to X850 specs was awesome! Same with the nvidia GeForce 6800GT being able to unlock to an ultra. Those were the simple days when they used the same chips and just had pipelines and shaders bios locked before hard locking came into play. Then of course the 6950 to 6970 unlocking was fantastic.
One really cool way of looking at old cards is this : archive.org/index.php
I've gone to newegg with that back when al of these cards we are talking about we're released. It's really cool to see how everything was back then with today's perspective. I love seeing the prices as well. It makes for a good laugh.