Right now I'm using the one that came with my Otterbox Commuter case. I am going to order SGP Ultra Crystal this week though. I've had good experiences with them in the past.
I used the power support shields (both crystal and anti-glare) for my iPhone 4S. But you can't order them on amazon unless you want foreign packaged versions. And they seem kinda on the expensive side.
I stumble across this really cool product called Rhino Shield its now on kickstarter (A project started by student in their bed room) it is not only protect the screen from from scratch, but also protect it from cracking.
I was really impressed with its impact performance, there is some really cool demo of the film.
Feels just like the original glass, if not smoother honestly. I would go for the glas.tr except my ADD coupled with OCD just wouldn't work with the slight piece of dust that I just couldn't get rid of.