I would go with opera singers as well. I have a good amount of experience in trying out a wide range of genres to sing in, ranging from rock to traditional songs from my country. I don't consider myself extremely good at any of them, but I believe that if you dedicate your time to one of these genres, you can become pretty good at it.
That being said, when I was a teenager, we set up a PA system for an opera concert, and the singer didn't even need a microphone to be heard. I think his voice could fill the entire venue (not a very large one, but still) on its own. It was truly a beautiful performance, and I can understand why people spend so many years practicing it.
Metal, on the other hand, can also be very demanding. It requires years of practice to achieve what these guys are capable of doing.
All in all, I believe that with enough time and practice, every genre is possible.
Cheers, Mile