I think the Belkin ones are pretty good. I've used that once for my 3G, 3GS, 4, and 4S. It's universal and has lasted me for over 4 years. I think it's like $7 at walmart!
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I guess i would want one where I don't have to remove my bumper. It's kind of difficult to fit the iPhone 4/S in the belkin arm band that i just mentioned.