Best Tonneau Cover?

It depends on what all you want to use it for and of course what you're willing to spend.

Diamondback makes a very heavy-duty cover with tie-down cleats to put stuff on top of it as well with tracks for crossbar racks, but is not something you're going to want to take on/off and carrying large tall items is not going to work because of the where the hinges are and how it opens.

Retractable ones like the Retrax and Roll-n-Lock are good quality and great deterents against theft and rain and you never have to (or want to) take it off. The issue is the canister for takes up some room so you basicly lose about 1' of you're truck bed although I have two plastic totes under mine that serve as toolbox so it I didn't really lose any room compared to a truckbed toolbox. You can carry larger objects with it retracted than with the Diamondback. I have the Retrax MX Pro which has a 500lb weight capacity and it's great for my needs.

Then you have the foldable ones like the ones mentioned here which are easier to remove and cheaper.

Real Truck was good to deal with.