Best tire with the highest ply for Prius C?

There you go. Check the treadwear rating.
There might be some trade-off with traction or rolling resistance though.
Nothing is easy!

 

The problem though with treadwear ratings is that they aren't an across the board standard. Tires from one manufacturer can generally be compared to other tires from the same manufacturer, however a yokohama with a 800 treadwear may be equal to a Michellin with a 400. I chewed through some 520 rated yoko's on my Mazda at least as quickly as some 400 rated Michellin's on another car.

 

Pretty much like the gas mileage ratings, those too are ESTIMATES.
But I think that your comparison of 800/400 is stretching the point a bit too far.
Comparing two different cars might not be fair either.

 

Aye, that's why it's a good idea to look at the treadwear warranty.