I wonder what the hardest songs in the game would be. If I had to guess, the hardest songs probably in the beatles discography:
Hey Bulldog
While My Guitar Gently Weeps
Back in the USSR
"Strawberry Fields Forever" (drums)
"You Never Give Me Your Money," while not overly difficult, has many layers and different chords.
"Because" is hard to sing.
The solos from "Taxman, "Your Bird Can Sing," "Yer Blues" and "While My Guitar Gently Weeps," and "The End," and "Something" would be the more difficult guitar parts.