I use this http://www.amazon.com/Speck-Products-CandyShell-Glossy-iPhone/dp/B005T0DV2K/
I like it because it feels like it offers good protection without adding too much bulk, and covers up the entire back versus having the apple logo open in the otter cases. You still need a front screen cover (if you need one). The one thing I don't really like is that you can only use skinny headphone jacks unless you have some sort of adapter.
There's a matte version that has a rubber coating which makes it more slip resistant.
I have biked with it in my pocket and it's fine