I am very experienced with all the iPhone Otterbox case designs (the defender series, commuter series, and now the Commuter series) because I have purchased all three for my iPhone 3. I would definitely have to say the Otterbox Defender Series Case is the best fitting, most protective, and most practical case for the iPhone 3G.
The commuter series is just too bulky, it takes away from the sleek slim look of the iPhone. My defender series actually broke a few months after purchasing from normal wear and tear, even though that series is supposed to be the best protective.
The first generation of the commuter series was a very good case. It was slimmer, while still being protective, but it didn't fit as well as I would like it to.
Now to the Otterbox defender Case. It just topped the other otter box cases by far. The case fits perfectly and looks great. It show the slim and sleek look of the iPhone 3 but still protecting it. With the hard plastic outer case it doesn't stick when you try to put it in your pocket or pull it out. With this case you can drop it on pavement without breaking (none from experience). Therefore, I highly suggest iPhone users to buy this case over any others. Don't waste your money on the other cases like I did and go with this one.