I'm really not much of a metalhead, but I'll share what I do like:
I second the person who mentioned Dream Theater!
Along those line, Mike Portnoy has some other stuff he's doing and it's all well worth the listen (especially if you like his work with Dream Theater)
Pain of Salvation might be my favorite band, period.
Coheed & Cambria, somehow not yet mentioned in this thread?!
Oh, and of course: Iron Maiden.
But I admit, I like strong, unique vocals as well as strong instruments. Screams and growls are generally not for me.
I know a lot of my metalhead friends like King Crimson. And I've liked what little I've heard of Nightwish (previously mentioned by someone else).
Hope it helps