For my 2015 GMC Sierra 1500 SLT, I've added the following carefully chosen and installed (by me) accessories:
Front & Rear Factory GMC Mudguards. To me, they have the best fit and protection. I flooded all fasteners and drilled holes with silicone for a rust-free install:
GMC Factory "Bed Rug." Made by Bed Rug for GMC, it is slightly less expensive than the Bed Rug aftermarket model, and it has a GMC logo to boot. Hidden, alas, by the Pace Edwards "Jackrabbit" tonneau cannister:
I had to modify the side rails of the PE Jackrabbit by 3/8" with a Dremel tool, as the thickness of the Bed Rug initially hampered the install. Tailgate now closes perfectly.
Last but not least, a "SuperBumper." Good for protection from a rear end crash, it will deal a severe blow to any vehicle that tries to get overly friendly: