I'm currently using Boxwave's ClearTouch Crystal. Got it for cheap and doesn't compromise sceen quality. The screen protectors that do compromise screen quality are the anti-glare and privacy ones.
Once on, I don't notice a difference, but it has a lot of finger smudging that I clean off on daily basis (just like stock screen). I also have an Incipio Clear Screen Protector which I haven't used (it came with the Feather Case).
There's also the more expensive Spigen SPG, but I think it's a little too expensive for my tastes. I've used Brando's $10-12 screen protectors that lasted me years (3+) and they can be reusable (peeled off, cleaned and put back on). Boxwave was my cheapest at $2.50 sale.