3DMark
3DMark is the latest version of this hugely-popular synthetic benchmark. Making use of DX11 features such as tessellation, compute shaders and multi-threading, it provides modern-day results and is available as a free download.
There's no question: Gigabyte's range-topping GHz Edition is the fastest single-GPU card we've seen to date. Heck, it makes Nvidia's reference board look timid in comparison.