I'd highly reccomend the ISkin Evo3. I'm quite impressed with this case. It's a pain to get on but you can tock with it on and it seems like it protects it pretty well to me. The clickwheel has a thin membrane over it to keep the dust and dirt out and there is a boarder around the scroll wheel when you put the case on which makes it super easy to run the click wheel in your pocket.
I second the recommendation for the Contour Showcase. I have one, and it's great. It offers the best protection of any other case I've seen, save the Armor, which offers no access to the controls despite its unparalled protection.
A third vote for the Contour Showcase. I have both of my ipods in Showcases and they look great, the horizontal orientation on your belt works out well, they offer good protection for all but the click wheel, and in my experience they don't scratch easily. Highly recommended.
I got an Agent18 Videoshield, which is a hard plastic (or polycarbonate) case. The screen does scratch easily, and there is no clickwheel protection. On the other hand, I've dropped it before without noticable results. Then again, if the case wasn't on, the iPod would probably have survived too.
What made me choose it over the Contour cases was that I'd heard the headphone jack openings on them wasn't big enough to get some plugs in. Dunno if that is the actual case or not.