Crystal will definitely reduce quality least. It will however show a lot of fingerprints and smudges. I just installed the Ultra Optics on my iPad 3 and am very pleased with it. It reduces screen quality a very small amount but reduces fingerprints a very large amount.
I currently have the Ultra Optics on my iPad 3. I had the PS Crystal on my iPhone 4S recently. PSC is clearer but UO resists fingerprints MUCH better and is fairly clear for an anti fingerprint screen protector.
If you want something that really keeps the clarity of the iPad, try the Spigen SGP glas.t. You're pretty much glueing an ultra thin piece of glass onto the screen, which in turn is as clear and feels like - you guessed it - glass.
If you want a more traditional screen protector, though, I'm really pleased with the clarity of the Spigen SGP Ultra Crystal, I actually put it on both my iPhone and iPad.