300 "austentitic" series stainless, specifically 309S, 316 or 321.
All have higher levels of chromium and nickle, for ultimate corrosion resistance. 321 adds titanium that improves thermal stablity at higher temperatures. 309S is rare and contains more chromium, but is often used in high heat applications.
Be prepared to empty your wallet, because any truly outstanding exhaust material is an alloy of expensonium and unobtanium...
Aftermarket manufacturers tend to shy away from accurate desciptions of the material composition, because the majority get away with "stainless" (which is true to a point) but hides the cheaper nature of the material typically used. Certainly by no means is all "stainless" created equal.
You either have to dig for the information, or call the manufacturer directly. Even then you might not get an honest answer or any answer at all, and there's no sure way to tell without an expensive alloy analyzer.
If it seems like a really good deal, it's probably not the top-end stainless appropriate for an exhaust system.
That said, anything is better than the steel tubing typically used. THAT said, stainless doesn't flow any better than plain steel (for otherwise identical systems), so you could perhaps buy four steel systems to get the same lifespan and cost of one stainless system.
Also keep in mind anything after a cat is a personal aesthetic choice, the only critical performance areas are items like headers and turbo downtubes.