For me, I have always had an Otterbox Defender. This case is not 100% dirt free, but it works very well protecting your iPhone. It is bulky, but I don't mind that. Also if anything happens to the case, peeling or even just dirt under screen, they will send another one out to you as long as you have sent them photos of proof of ownership. If you want one that's fully dirt free, the other Otterbox cases are, but cost more.
If you want one that's waterproof, I literally just saw a commercial for the Lifeproof case. I've heard it is good.