There are assorted threads that provide reviews of each product, but I don't believe there is a ranking of any sort. If there were, I'd say Invisible Shield would be at the top, but that's my opinion. I've also used BestSkinsEver. While an ok product given the price, I don't think it's in the same class (or price) as IS.
I haven't used BodyGuardz, so couldn't comment on that either. Here's my Invisible Shield for iPhone 3G review and install video. You'll also find reviews from readers of the site.