I actually have 2: The HD 4800 series, particularly the HD 4890 and the R9 200 series, particularly the R9 290x.
When the HD 4800 series came out, it was kind of like what Ryzen did to Intel. AMD came back punching offering 80%-90% of the performance for about 50% less cost than the competition. I was so pleased with it and was so happy to see a red card in my system. The HD 4870 was also the birther of GDDR5.
When the R9 200 series came out, it was just pure awesome. They were so powerful and the 290X actually took the crown for about a week for the fastest single GPU out there until the 780Ti came out.
For me, that would be my HD 5870. It was faster by leaps and bounds than NVidia's offerings, and was the first GPU to offer Dolby Digital True HD and DTS master pass through via HDMI. I had been considering a high end sound card for that support prior to launch $250, and then AMD came a gave that plus amazing gaming performance for the MSRP of $379.