What is the best HEAVY METAL speaker?

I'd stay away from horn, electrostats and planners.

IMHO, metal needs critically-damped, very articulate dynamic (cone) speakers driven by solid-state that can really control the speakers. You can lessen the requirements of the amp if you high-pass the speakers and use a set of servo-subs.

A good track to test a "metal-worthy" system is Tool's "The Grudge". Listen to the accelerating kick drums. Each impact should be well defined (and not a muddy mess). Vocals should sound distinctly layered on top of the sonic fury. The engineering / production isn't perfect, but if you can get this song to sound "right" you're moving in the right direction.

"... if a speaker is really good at classical music, it will be good with metal too."

I kinda disagree. IMHO classical is very a forgiving music genre (from a tonal perspective). I've heard classical well-reproduced on Quads and Maggies and Martin Logans. I've never heard metal well-reproduced on any of those speaker brands.

However, Fields of Nephilim is a good reference. Same with early Husker Du (or any Husker Du, really). Awful engineering. Awful production. But a great metal system will allow those "awful" qualities make sense and not detract from the musical genus.

Well, I guess classical is only well-reproduced on dynamic cone speakers, because I've never heard metal well-reproduced on anything other than that. It's gonna break the hearts of Quad and Maggie owners that their speakers aren't that good.