Seems I've never posted in this thread. I've listed some of my favorites below and linked the name of each artist to a song on YouTube (embedding this many videos would have taken quite a lot of space) so those not familiar with the singer can have some kind of reference. I tried to pick videos with good sound quality, but couldn't find anything in a couple of cases.
Also, if Vocaloids and such are allowed, then these ones as well:
I love Tarja Turunen!