Of course, it all depends on how you use the Touch - and what kind of protection you require.
For example, I don't like thick, silicone or plastic cases, and the flip variety just adds to the thickness. Unless you're in the habit of dropping your Touch, there's not much chance of breaking the screen - though it's been known to happen, rarely. The ultra-thin Bodyguardz skins affords 100% coverage of the surface against scratches and light bumps, and the surface resists the buildup of oils.
This list contains all the major vendors, with reviews:
http://www.ilounge.com/index.php/reviews/reviewgrades/search&keywords=Touch%20case&category=286&order=entrydate+desc/