Back when I had my S4 my case of choice was the Poetic Atmosphere. I like how it showed off the back of my phone, and kept a thin profile.
Here's a video review of the S4 case.
Now they're coming out with an Atmosphere for the S5, and I'm excited to see how it looks with the new dimpled back on the S5. The pictures don't show the real deal yet, but hopefully there'll be new images up soon.