What is the hardest song you have learned or are learning on bass?
I would say personally it would be Metropolis pt1: The Miracle and The Sleeper by Dream Theater. That song is Insane. But thats just me.
im trying to learn The Rime of the Ancient Mariner by Iron Maiden.....all the changes of rhythm and tempo is tricky
Schism by Tool. It's made to be played by a man (thicker fingers helps alot in this song). Thus very frustrating to get it down and at the right speed. Also TotalImmortal by AFI is complicated for me to learn. 
With Schism. Justin Chancellor uses a pick. Its a little harder to do, but it gives you the right tone.
Learnt Death Waltz (Touhou version)once, boy was that ridiculous. Don't try unless you can tab or read piano, because there is nothing. I mean NOTHING on it, not even a melody. I posted a cover of it after 1 1/2 months of practice (working out inclusive) and it's not the greatest 
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Freaking "A devil in a midnight mass" by Billy Talent. It's not hard, It's just I can't get the bass intro right. I always start too early or too late...it's annoying me. -_-
