The Defender Series used to be OtterBox's toughest case, but now the waterproof Armor Series, which comes in three different colors, is the company's toughest. I like the slimmer design of LifeProof's waterproof case better, but the Armor is a beast of a case and you can access the headphone jack without using a special dongle. That said, I didn't entirely trust the seal on the little door that covers the headphone jack. But OtterBox says the case is rated to be underwater as far down as 6.6 feet for up to 30 minutes.
While Armor case lists for $99.99, you can find it online for around $75, which is about what the LifeProof costs (Note: OtterBox recently bought Lifeproof, but for now both brands will coexist). A version of the case is also available for the iPhone 4/4S.