No, I think you mean a true flat tax, e.g. a poll tax, is the ultimate regressive tax.

I have come to find that "regressive" moves the goalposts. It sounds as if the rich pay less. They don't pay less; they pay more. They just don't pay more when dividing taxes by their total income. I've never heard a convincing argument for why that formula is the ultimate benchmark.

And anyway, virtually all sales tax proposals have some per capita rebate/basic income, and that can be tweaked to make it as progressive as your heart desires.