Since it doesn't get put into a pocket, I almost skipped it on our 12.9/Gen3, but I also have some travel plans where I'll be using it, so figured, some scratch protection couldn't hurt. I went with JETech, a brand I've used in the past, 2-pack for $15, 4.4/5 @ 1100 reviews on Amazon - these are glass, so they really preserve the feel and clarity of the OEM surface, perfect fit, easy to clean.
FWIW, I went with a TineeOwl brand case, it's pretty minimal, decent fitment, TPU edges with a clear acrylic type rear, pretty solid combo for protection while keeping the device about the same size. Oh yeah, and the Pencil comments nice and strong (the charge side is flat to create a good contact area).