Nillkin Super Frosted Shield Matte:
Minimal case, but with a slight lip around the glass and camera bump protection. Full 360 degree coverage with tight cutouts and good buttons. A very tight, accurate fit; it won't bounce off if you drop it.
Covered with a bump pattern to make it grippy. Hard plastic that will likely crack on dropping.
For: people wanting to reduce the slipperiness of the phone, make the bump look less dumb, and protect from scratches and light drops.
Spigen Liquid Air:
Light by Spigen's chonky boi standards, this gives protection not far off their Rugged Armour series without the goofy carbon fibre accents. Looks good and fits great. Large protection lip around the screen and sufficient camera cover. Tasteful design. Not as grippy as you'd think, but better than just the phone.
For: serial phone-droppers who don't want to make their giant phone any more gigantic. A good compromise case.
Official "sandstone" bumper case:
More rugged than the Super Frosted Shield, but with openings for the rockers, buttons, and top and bottom.
Extremely grippy texture and a tight fit. Good screen lip protection and camera cover. Aesthetically a bit weird; a bit "HTC Desire era" with the big gaps all around. Tasteful OnePlus logo.
Colour and texture of an asteroid space station wall; really needs to have "REMEMBER THE CANT" spray painted on it.
For: official case fans who don't mind paying twice what it's worth (unless you find it on eBay). Daredevils. People who like to click the proper buttons.
All three are good, but if you spend the RRP on the official case, you should go get your face paint on and get to the circus, because you're a clown.