I use the Ford Genuine, Super Duty branded mat (HC3Z-99112A15-A):
It's great. Fits perfect. Looks native to the truck. Very thick. I routinely drop logs into the bed from an excavator and the mat easily absorbs the impact.
I have this one too. For my own needs i actually cut it into two pieces. Cab side stays in permanently, tailgate side i use when necessary. I like that it says superduty, no other mat does. Works great.