I use an otter box defender. It's very bulky and not for everyone, but I'm a pretty clumsy person and I enjoy having peace of mind. I'm pretty sure this case could withstand a small firearm.
Otterbox makes some of the highest quality cases. This one has a thin piece of plastic that covers the screen in addition to a hard plastic skeleton that encases the phone, as well as a gelatin like casing that wraps around the skeleton. I've dropped my phone a few times and split coffee on the screen too. My phone has survived it all without even getting touched.
The screen protector doesn't interfere with operation of the phone, and you can't even tell it's there. Sometimes it does give an "oil slick" appearance, but you can massage those out.
Some people use their iPhones without any cases or screen protectors, and as long you're very careful you should be fine. The screen is really durable, but you may get some scuffs on the case.
Another option is the bumper from Apple, which just wraps the outside edge of the phone. Really minimal protection but it may protect it from short falls. Another option is a solid case that wraps the sides and backs of the phone and leaves the front open.
