Generationally:
Pre-DX7; Voodoo2 12MB. Has there ever been a more iconic graphics card? It even ran modified Doom3...
DX7; Geforce256 DDR. A revolution in its own right, first chip with hardware T&L. Honourable mention to the Geforce2 MX. Might not be the greatest card ever, but it did introduce a lot of people to PC gaming.
DX8; Geforce4 TI4200. Amazing card for the time.
DX9; Radeon 9700Pro (R300). This chip practically invented DX9 gaming, and DX9 for that matter since DX9 was practically written around the R300 chip. Think about how important DX9 has been since, everything from smartphones to highest-end GTX TitanZ all support DX9 as a base-line...
DX10; Geforce 8800. Arguably one of the longest lived newer cards out there. Just shows how sound its architecture has been.
DX11; Radeon HD5800-series. First DX11 chip. Launced in 2009, and still a perfectly viable gaming card today, 5 years later.
I do think the HD5800 was one of my all time favorite design from AMD, I would say it was extremely surprising. So for me it goes
ATI- 9700 (r300) to HD 5800 as two monumental designs which stood out from everything in between.
But i am not sure the HD5800 is the defining chip of DX11. It was the first DX11 chip and that is part of why i think it is so relevant. But the 5800series was very impressive on so many fronts. To me it is more like Nvidia's 8800GT because it was really good everywhere including price. The whole package, very impressive
But AMD couldnt use the HD5800 architecture for as long as Nvidia used their DX10 king. This is because if AMD wanted to get real serious with DX11, they would have to dramatically change the architecture.
So while i love the HD5800 for all that it was, I wouldnt say it was the king of DX11.
For nvidia, i would have to say from 8800 and.........
idk, its hard to say.
Fermi wasnt without some major problems but it set them in the right direction. It rolled out the carpet for kepler which was a fine architecture but nvidia took so long rolling the full kepler line out that it never had a grand slam. Had the 780ti come out when the gtx680 come out, then that would have been for sure the next 8800 but it didnt. Nvidia dragged their feet so long with coming out with a real gaming gk110 in all its glory. So long that AMD was able to pretty much catch up with hawaii.
So while Nvidia has been keeping the bases loaded and playing real well, they havent hit a grand-slam like the 8800 every since.
Had Nvidia gave us a 780ti at the time they gave us the 680, it would be a different story. And titan........it doesnt deserve to be on this list