TopACE Clear-Gel ($8)
TopACE has the best reviews on Amazon so far for the HTC U11. Being one of the first case manufacturers to offer a case with the rear microphone cut out. Yes, this is a big problem where most cases need to be returned and redesigned.
The TopACE Clear-Gel case is completely transparent, yet feather-light. Meaning you'll still see the color-changing design of the rear glass on the phone, while still being fully protected from accidents.
If you're looking for an ultra-thin TPU case that will keep the phone safe without adding bulk, this is it.