the best of everything
The ATS is fantastic. You can tie it off like a Piranha. It is small and simple like an ATC. You can flip the curvature of the device to vary the friction levels, which comes in very handy. You can also use it as an auto block, which I don't believe you can do with an 8, Piranha, or ATC. Rappel racks are probably the smoothest overall, but they are too bulky and heavy for backpacking.