I am a dancer.... Not a singer! Every song would be difficult for me to sing and even MORE difficult for you to hear!
In your opinion, what song would be the most difficult to sing ? ~ and why ? ~
A different language.
A different language, or a song that you don't emotionally understand. Or literally, for that matter. But any song above or below your vocal range is super hard to sing. I am a singer, (a hobby not a profession) and I find it super hard to sing a song below my range. But I'm learning!!:)