IMO Apple Silicone hands down, without a doubt is the benchmark for form factor. It forms to the 17Pro almost exactly with the continuous round edges. For me form wise closest to holding a naked 17Pro. And the weight is probably a factor too.
The only problem is its silicone. To me the Caudabe Sheath 15 Pro was the closest to the Apple Silicone in form and specs(not weight), without being silicone.
The only problem is its silicone. To me the Caudabe Sheath 15 Pro was the closest to the Apple Silicone in form and specs(not weight), without being silicone.