Spigen Tough Armor Case. I had this case for my iPhone 5s and loved it. This case doesn't add a whole lot of bulk, but I feel like it sufficiently protects it at the same time. Slap a glass screen protector on it and you're good to go!
Tough protection like an otterbox, keeps slim design, plus decent "tactical" look. I have dropped my phone many times on the pavement getting out of the truck and not a scratch yet on my phone *knocks on wood*
I ordered a couple of these from a small company in Indiana that makes them by hand. They have a lot of options. They're made with real wood, and the rubber sides have a nice grip. Not too bulky either, and most of them are no more than $30: