my example of a good shredder would have to be joe satriani. His understanding of harmony and melody are incredibly difficult to top, according to me. Also, his incorporation of multiple cultures in his playing never stops amazing me. I think the main thing that distinguishs him is the way he can fuse styles and sounds and still sound amazing.
In all honesty, i don't think i've heard a bad shredder- but maybe thats just cuz im jealous of people who can play that fast...