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The best if you count max memory clock is ASRock X99M Killer 3.1. The best if you count max CPU and still high memory clock but full atx size is ASUS Rampage V Extreme ( can be 3.1 ).
Personally I'm benching on ASRock as it's overclocking CPU the same and memory goes higher. It's also smaller, cheaper and seems more solid. ASUS already had typical "can't see memory" issues and went to RMA on 00 error code ( couldn't see CPU ).
Most X99 boards should OC CPU about the same and differences are in memory overclocking and additional features. Still most boards should handle DDR4-3200 while max possible on 4 memory slot boards is ~3466.
If you are planning to overclock on air/water then don't spend too much as base for all X99 motherboards is the same. As EarthDog already said, check if board has enough additional controllers etc.