What is the fastest i7 Processor?

I would like to know which i7 processor is the fastest without overclocking. I see different types of memory that each supports...such as 4820k supports 1866 and 4771 supports 1600. I am running 2400ghz XMP memory. I am trying to decide if the 4771 on a board that says 2400 (overclocked) will be faster than the 4820k on a board that says 2400 (not overclocked). Which processor is actually the fastest? I have been told the Z87 chipset is the fastest but all the processors and boards are overclocked at 2400.