I have been using Helium screen protectors for almost a year now on three different iPhones and they are the best. The fit is perfect and doesn't smudge too much and is crystal clear. You can hardly tell it's there.
I have used most of the popular brands and none of them have held up like Helium, and it's also the least expensive at 3 for $12.00.
I think my shortest lived and most expensive screen protector was the zagg, 2 sets wasted, although it made my iPhone a little more slip resistant, it peeled off when I put it in my jean pocket (and I don't wear very tight jeans), and one peeled off when I wiped it on my shirt.