I honestly do not find anything to be very difficult to play. When listening to a cover, it is sometimes difficult to figure out what is being played, but I'm at the point where I can play everything I have ever wanted to and am bored because I cannot find anything challenging to play. The most difficult song I've had to learn is The Real Me by The Who but I can play it with ease. The hard part was figuring out the notes, playing them is the easy part.
Agree with this, I'm working on The Real Me right now and I'm definitely not fast enough yet. Makes The Trooper easy now. As far as figuring out the notes, there is a guy with a youtube who plays it note perfect, so I try to watch what he does and am using Songsterr a little bit.
Overall, I found Zeppelin songs more difficult to play than others. Black Dog is easy to remember, but just fast. On the other hand, The Lemon Song is near impossible to remember, and not easy to play either, although my first year playing bass I was able to get to the point where I could play the first 3 minutes note perfect, too bad I can't remember any of the riffing anymore. Traveling Riverside Blues also gives me a lot of trouble, it's fast and the timing is difficult.
Metallica Master of Puppets and That Was Just Your Life are extremely fast as well and wear my left hand out pretty quick.
That is a fiendishly odd tune to play well! Great song but not for those who can only play 3 chords! That's what set the DK's apart from a lot of their contemporaries.