hmmm, well I agree with what someone said earlier, as you learn a song, it doesnt seem so hard anymore, its only beforehand, and practicing that make it seem hard, except for the odd song that you just can't get perfect...

Opeth - Benighted, and Face of Melinda
They're probably the hardest ones I know, I also know Night and the silent water is pretty hard, but thats only because its harmony almost the whole way through the song (10 minutes or so) and some of the string bends those guys do in time with each other would be a bitch to get right with someone else, yet you cant play the whole thing on your own unless you just record it (boring)