7.1. 7.5 seems only really useful for 68000s and 68020s that can't run 7.6 but need close-to-7.6 features (and was a minefield to boot), and 7.6 seems unnecessary if those machines could just run 8.x, but 7.1 can be made to do nearly everything 7.6 can and runs great on old machines.

That said, I probably wouldn't run 7.1 on a PPC unless I had a very good reason to. None of the PPCs here have less than 8.6 on them.