Do you prefer a hard case or a rubbery one or a leather-look one?
I like the hard, slick cases because they slide in and out of my handbag (or pocket) faster and are very tough. On the other hand, they do scratch up more easily and need to be replaced if you want to keep them looking sleek.
If a hard case would suit you, I like the Capsule Rebels (in plain colour, not multi-coloured) and the Griffin Reveals. The Reveal is slimmer, but not as tough, and has a coloured rim with a see-through back. Griffin has a bunch of other styles for all models of iPhone. I've always found them to be high quality, have been using them since I got my first iPod.
http://www.switcheasy.com/products/Rebel/Rebel.php
http://www.griffintechnology.com/products/reveal