One of the first songs I ever learned is still one of the hardest for me to play. That would be Led Zep's 'Black Dog'.
In and of itself, not the worst or most physically taxing song to play but to play it right can be a bit of a challenge. To play that riff that appears between the verses and do it right takes lots of practice to get the timing down. That is to play it in the groove as the original. I remember learning it and boing happy that I could play the riff. Then I decided to play along with Page and realized that it was a pretty tough riff to get right and in time.
So, just a thought. It may not always be the technical wizardy that makes it a hard song.
