i like the zagg ones for my previous phone but they are a bit thick and i'm not sure how it affects a touchscreen. They are basically impossible to scratch though. My N900 was used and it came with some cheaper ones that really scratch up easily. Not sure the brand but i'll check later if anyone really cares.
HORI makes some really good ones too, but they are only sized for a PSP or DS as far as i know. But if you cut them down to size they will work fine, I've used the PSP one on an n800 without cutting I think.