The 6600 didn't have general purpose registers. It had operand, address and increment registers. The 801 development began in October 1975. The Cray 1 had been in development for 4 years when it was released in 1975.

So I'll go with the Cray as being to OG RISC. But yes, firsts don't really matter except to say it sure wasn't the Berkeley RISC or the Stanford MIPS.