Tough question. I've got a pretty substantial Datsun collection and have restored many more. My pristine '73 Z still holds the record for highest-selling '73.
With that said, you didn't specify a year or model, but the earliest arrivals wore Bridgestone Super Speed 20. My '73 had Toyo tires. I have seen cars as early as 8/70 with Toyos, and cars as late as 11/73 with Bridgestones.
Per Carl Beck, who has forgotten more about early Z cars than I'll ever know:
"The original tire spec. for the North American Market was an 175SR14, but because Bridgestone could not supply enough tires to meet the demand from the US market, they started shipping cars with 175HR14's and Toyo became an additional supplier. For the Series II cars the 175HR14 became the standard tire spec, from both Bridgestone and Toyo."