What top 10 songs in your opinion are the hardest?

Just out of curiosity, what top 10 songs in the world do you think is the hardest? If you could, could you add why you think they're hard to sing? Thank you in advance :-) This is not in any specific order, it's just the way they are popping on my mind.

Whitney Houston - I Will Always Love You is pretty hard to sing. You need lots of power and belting intensity to hold the notes in the chorus.
You said "in the world" so I assume I can put some opera stuff, too.
Queen Of The Night - Aria from Magic Flute by Mozart. Just go to youtube, you will realize why it's so hard when you hear those extremely high notes.
Christina Aguilera - Fighter. It's not that it has too many high notes, though there are some, but it also demands good control, and strong voice to do it properly.
Probably Without You by Mariah Carey, too, and My Heart Will Go On by Celine Dion, since those are pretty demanding songs.
And another one that comes to my mind is Beautiful by Christina Aguilera - That is if you really want to hit all of those high notes. There are some pretty, pretty high. (Sun will always, always, shi-I-I-ine).

That's about it, nothing else pops to my mind. Opera, in general, is really hard to sing since it demands superb vocals. And in the mainstream, well, good vocalists make hard songs, so, look into that.