I would have to say the most difficult song I learned was "Seek And Destroy" by Metallica. I was just getting serious again at playing guitar back in 2003 when I met this guy who had a band and was looking for a rhythm guitarist. He was BIG into Metallica and he wanted me to learn the rhythm part of the songs he had already new from that band. It took a couple of weeks for me, to be honest, to get that song down pretty good. I would have to say that the song "ONE" was also kinda difficult for me at the time also.
