Depends on what you're doing with it.
Line-X is the hardest one out there that I know of. great for trucks with open bed and exposed to the elements in truck beds that actually get used.
Rhino is one of the softest I think, and more sutable for something that is covered, for example a tonnoe cover or shell. It's easier on the knees when crawling around in the bed.
Ultimate linings is somwhere in the middle.
HTH,
PSD