Wow, I'm surprised. Arch seems to have the ideal "prog metal" voice, for my money, anyway. Extremely virtuosic, with lots of unique phrasings, vibrato, and syncopation that can stand toe to toe with the complexity of the instrumental music rather than having to make it step to the side and give the singer room. It's also piercing, which is important in prog metal because you've got some very busy pieces of music with lots of loud, crunching guitars that you need to be able to cut through without cramping them. Aside from that, I love the sound of his voice, and he's a first-rate melodist to top it all off.