I'm not really sure what is meant by 'powerful', so I'll just list some of my favorites:
Hansi Kürsch (Blind Guardian) - I shouldn't need to give any reasons. He's just a billy bad-ass. He's got a completely unique 'metal' voice (how the hell does he get that rasp??), and a beautiful clean voice. Then, when you add forty other Hansi's to the mix, well...
Vintersorg (aka Andreas Hedlund) (Vintersorg, Borknagar, ex. OTYG, among others) - Another singer that is hard to touch. A snarling black-ish scream, but a WONDERFUL clean voice. Just listen to any of his albums. If you want bombastic power, listen to Hedniskhjärtad, Till Fjälls and Odemarkens Son (don't know how to do a umlaut-capital O). Such a powerful baritone sound. If you want a more natural sounding voice of him, listen to Cosmic Genesis through The Focusing Blur. Very, very creative vocal lines, perfectly executed with feeling.
Jonas Renkse (Katatonia, ex. October Tide) - You can take the early Jonas (aka, Lord Seth, Lord J. Renkse, hehe) or the newer Jonas, it's all great. The early screaming Jonas was full of raw emotion - he sounded like he was screaming tears. From Discouraged Ones to present day, we have a tearful clean voice, sooo emotional. He's become quite an accomplished vocalist, able to hit notes right on key. There's no one that I've heard that is able to convey that sadness, that desperation, and sometimes, that fear, quite like Jonas can.
Mikael Akerfeldt (Opeth, Bloodbath, Katatonia briefly, among others) - Quite a versatile vocalist. He's got kind of a folky, mellow clean voice, but a deep, evil death growl. The way he can immediately drop one in favor of the other is impressive. I'd say he's got the best death growl of all the growling/screaming music I've heard (which, admittedly, isn't all THAT much).
Russell Allen (Symphony X) - There's no need for me to explain.
Warrel Dane (Nevermore, ex. Sanctuary) - A very unique voice. The main thing I love about his singing is that half the time it sounds like he's losing his mind. Very, very raw, but full of emotion. At times, you could say it sounds overwrought, but that's part of what I like about it.