The two songs I've had the most trouble learning so far: "Number of the Beast" -- just the intro & outro riff. I've got it down now, but when I first started the project of learning a bunch of Maiden songs, my picking forearm would cramp up about halfway through the intro ("Just what I saw ...") from all the rapid downstrokes. I wore a compression sleeve on my right arm when practicing it for a few weeks, and finally my stamina built up and I can do it with the band no problem. Fortunately, as Dave, I get to set the tempo and I'm not at the mercy of the drummer. "Caught Somewhere In Time" -- that fast gallop is another forearm muscle killer. And the lead is tricky, the chord progression underneath is weird and changes rhythm a couple times. That's fortunately not on the setlist for my first gig with the band, because I need to work on it a lot more before it's ready.