Leather is good.. I loved my g4 for it's great leather removable backs... and all of last 4 phones have been in a Pierre Cardin leather case.
Metal is OK and can come in my different finishes too.
Glass.. Never again.
Rubber is also a good choice, provided it is done correctly.
Plastic is also acceptable, because of all of the ways of doing it.
Carbon fiber well laid up and executed properly would also be nice... but very expensive and time consuming.
What I want to see, is a removable back that has the gasket around it to improve it's water ratings and for manufacturers to actually treat these phones with a substance like Liquipel. Even if they charged as much as $50.00 more it would be worth it.
Mac