In my opinion you can't get better than boomerang tags:
Boomerang Pet Tags: Guaranteed to last. Mailed today, Free Shipping.
They are guaranteed for the life of your pet- if they ever fade they will send you new ones- but they won't fade. I have had Armani's for 3 years, Chloe's for 1.5 and they still look brand new. I think I paid about $10 for each tag, no shipping, no tax- they are engraved on both sides.