Tarja was good, but I think the others have been good, too.
Still, Tarja with Sharon Den Adel (Within Temptation) is an AMAZING combination:
I think my absolute favorite Nightwish song, and possibly favorite song of all time by anyone, is The Greatest Show on Earth, from their latest album (Endless Forms Most Beautiful):
It is a 25 minute song about the creation of the universe and humanity, and people who aren't into science won't get a lot of the references. There's also a nice spoken bit, though I'm not fond of the man doing it (Richard Dawkins... and not because he's an atheist, but rather because he's kind of an *******).
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That sounds promising. I came across some Mongolian music a while back, it was very interesting. Wouldn't mind finding more.