Generally, you get what you pay for... Fastest AMD CPU based on single core performance is the Athlon 860k, which is basically an A10 7850k without the integrated GPU. Nowadays, games are starting to use more cores, so, the FX-6300 will outperform the Athlon 860k, and any FX-8 will outperform the former two when things are working properly.
Intel has AMD beat significantly in single core performance. In i3 4160, practically the slowest modern Intel CPU, has 30% better single core performance than the Athlon 860k.