Which kind of iPhone case is the most protective?

Silicone, hard cover, leather (like Apple's), or silicone and plastic (like Speck)?
Also, with silicone cases, do they ever come off your phone when you're taking it out of your pocket? I had a Belkin one for my iPod Touch and it did that, wondering if iPhone cases do that too.

The most protective Iphone case that I ever used when I had mine was the Iphone Griffin Survivor Case.
It lasted me right up until I gave my Iphone to my friend and she still uses it now, It's Shock proof, dust proof, mud proof, snow proof, Water proof, you name it. You can get them in many different Colours and they have them for all the Iphones. I bought mine off Ebay for around $45 and it was so worth the money. If you do not use ebay then you can always order them off the Griffin Survivor Website or even check out your local phone store and ask them about them. Good luck.

These are some videos of how strong the Case actually is. Seriously Check them out.
Snow

Water

Thrown Off a bridge