Just one band?
Allright, since Opeth is allready mentioned I'll have to go with
Catharsis, an American metal/hardcore band. Their destructive anti-establishment and self searching lyrics are just incredible and fit so well with their near blackmetal and hardcore mix. The music is so destructive, but it also has a strange beauty thats hard to describe... While it truely describes the darkest sides of our society and our own existence, it also shows that there is still hope. It really is exactly what the name of the band says, a catharsis.
Too bad they're on a hiatus at the moment, and I doubt they'll ever come back togeather again, but you can still check them out on:
www.oddshaped.com/catharsis