Strongest Case

I believe the Otterbox has gotten kudos for protection.

http://www.otterbox.com/iphone-cases/

Hmm .. not finding them at the Apple Store now. Not sure what that means.

I use an Agent 18 Slim Ecoshield (I don't think it would win any awards for impact protection) and did use a Switcheasy Capsule/Rebel

http://www.switcheasy.com/products/Rebel/Rebel.php?color=black

Better protection but the Power (top) and Volume switches are covered which I disliked.

Phil

Message was edited by: w7ox

Re the Otterbox:

I believe it was sold by the on-line Apple Store but I cannot find it now. You may want to verify (either by a post here or by calling Apple) that it will work OK with the 3GS. I'm thinking it might interfere with sensors or some such thing.

Phil

Speck makes a nice, heavy duty thick rubber case that looks like it offers very good cushioning. I wouldn't consider anything "drop proof".
My biggest concern is balancing protection with cooling. My thick case may trap heat and I live in South Florida where it gets Hot & Humid.
Does a thick case block proper cooling and do I run the risk of overheating my phone?
I'll see if it ever develops into a problem.

The Otterbox Defender works perfectly well with the iPhone 3GS. All sensors function normally. You can get it a lot cheaper on Amazon.