a lot of the shredding songs are ovbiously thharset songs due to their speed, and metallicas 'Blackened' riff took my a little while to get the hang of. but for me the hardest songs are classical songs. i'm currently learning Violin Prelude by J.S. Bach , and trying to get fingerpicking wrapped up, as that is real difficullt for me, so i'll need to practice more on that.

if you do feel that the fastest songs are the hardest, then i suppose some of Michael Angelo's shreds would be extremely difficult to play.