Best motherboard with at least two pci-e full 16x slots.
Better question, which platform are you wanting to use, what are you using it for, and are you overclocking?
@MajorLacasse Yeah, if you plan on building a workstation server with 4 Firepro cards for insane OpenCL rendering. We need to know what the purpose of his build is first before making massive recommendations like that.
Best? Asrock Extreme11 X79.