I mentioned before that putting the gorgeous iPhone 4 inside a case generally fuglifies it. So I prefer the iPhone naked or wearing a Sena Ultraslim Pouch. And for the times when I do need a measure protection I use a Speck PixelSkin.
But if the going gets tougher, the only answer, at least previously, is a heavy-duty case like the Otterbox Defender. Unfortunately, it costs a lot, P2,500 the last time I checked, and it has an ugly circular cutout to show off the Apple logo (why?!?). So I searched and found the Speck ToughShell.