If you prefer the feel of going case-less, or you want basic protection for $5-$15, then stop reading now. But if you want a case that does it all, including total drop, dust, and screen protection and allowing for swappable modules (including tripods, lenses, backup batteries, credit card readers, and more) to be attached, then an OtterBox case might be just the thing for you. It's available for many brands (including, Samsung, LG, HTC, Motorola, OnePlus and more), but likely will appeal most to iPhone users. Depending on which model case you choose, they range in cost from $15 to $150.
Link to OtterBox Website