The first x86 implementation of UEFI was Insyde Software’s InsydeH2O, and the first system to use that was Gateway’s 610 Media PC, which shipped in late 2003. So technically, the first PC motherboard to use UEFI was the motherboard in that system. (It did however boot Windows in BIOS mode.)
OpenVMS added support for UEFI in 2003 so there might have been Itanium systems using UEFI earlier than the Gateway system. (Itanium systems initially used EFI, UEFI’s predecessor.)
As far as I can determine, the first widely-available UEFI motherboards were Intel’s 945 motherboards, e.g. the D945PAW (May 2005) or the D945GBO; their POST codes show EFI boot phases. These still relied on BIOS boots; full UEFI boards came later, and some are listed in a 2009 UEFI whitepaper.