What is the toughest dog chew toy?

I have 2 dogs, a pittbull mix and a american bulldog mix. Just the other day we spent over 75 dollars on new toys for these guys. And today they are ALL demolished!!! We need something that is VERY durable. We've tried the kongs, tires, ropes,even a toy that was made from bulletproof plastic. And every single one of them the chew to pieces!!! I mean literally to pieces. Do you guys know of any toys that would last longer than 2 days. I'm tired of spending money on toys for them and then coming home and finding little bits and pieces of what used to be their toy. And then sometimes not being able to find the toy at all. But to be surprised a couple days later when its puked up on my carpet. Please help!!!!