I use the otterbox commuter. It's not so bulky like the defender. So far it protects the phone, lent just gets under it from being in my pocket most of the time.

http://www.amazon.com/OtterBox-Commuter-Series-Case-HTC/dp/B00BJN7BPE/ref=sr_1_3?ie=UTF8&qid=1393179528&sr=8-3&keywords=htc+one+otterbox+commuter+case