What are some of the most difficult songs to sing?

I think It might be fun to see what everyone thinks are the most difficult songs to sing out there. Here are a couple I think are just insanely hard:
https://www.youtube.com/watch?v=IwzUs1IMdyQ
https://www.youtube.com/watch?v=SQNtGoM3FVU


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Black Rain
Beyond the Wheel