Its very hard to tell. The 3960X is the best assuming all six of its cores can be used and its Sandy-Bridge-E so it overclocks better probably due to better heat dissipation. However, on a clock-per-clock basis on quad core, the 3770K is the fastest. Although once again...you can probably overclock a 2600K and 3930k more. All of the CPU's at stock clock are enough for gaming anyway.
Check Anandtech's benches and you can see a 3770 exchanging blows with the 3960X on a load of benchmarks.
i7-3930 is a great hexcore CPU if you are willing to spend +300 for the CPU, +200 or more for the motherboard, +50 for quad-channel RAM sets...pair it with an Asus X79 and you get 3x SLI on PciE 3.0 at 16x/16x/8x. Overclock it to 4.5 Ghz and you wouldnt notice the 100mhz less than a 3960x which is $400-500 more...and 12MB L3 is still enough even compared to 3960x 15MB.. the money you'd save could go to SSD, better GPU, etc.
With the temp issues with Ivy's, most people stick with Sandy for the overclocking unless they need Ivy's HD 4000....