What is the best HEAVY METAL speaker?

For a larger room, any of the bigger Legacy speakers will work well (Focus SE and up). Though the Legacys have fairly high efficiency ratings, they like a lot of power. For a small to medium sized room, the Vapor Audio Arcus paired with a good sub would be very tough to beat. I own a pair of Arcus and they will play down to the mid 40's at any volume I can withstand in my room and not fall apart. The subwoofer will give that low frequency lock that metal music requires. Arcus have some of the best midbass of any speaker I've ever heard and will play it to extreme levels. These recommendations are based on if they fall within your budget, obviously.

Any of the old Polk Audio speakers that have the multiple 6.5 inch woofers and the passive bass radiator. They are the best bang for the buck and can handle all the power that you can throw at them. Just ask anyone that has ever owned them and they will agree that these speakers can rock.

As some one who has been into this hobby for 20 years and started out listening to metal (still go some times) you need to stay away from razor flat speakers, Revel and Thiel are good examples. They sound good when the music has bass recorded in but button up quick when the recording sucks.

I have always liked Wilson Audio and B&W for metal. Their flaws of over done bass thumb (upper bass 80-100hz) lends itself well to hard rock heavy metal. Older models of both brands can be bright so stick with newer ones (sophia 3,sasha or later, 802diamonds).

You can always add subs and a good digital highpass but that is not cheap if done right.

Wharfedales. even the Diamond series handle metal just fine.

I do not know your Budget, however, you asked what speaker would do what you ask the best, first off, the sensativity needs to be good, and you need alot of air movement for woofer size, here is a speaker that has fast woofers and a 96 sensativity rating, you like double bass, well then here you go, The JBL DD-Everest 67000 speakers, they have 2 15" woofers, do 28hz to 50 khz, they are horn speakers, so you get the the impact of metal music the way it was meant to sound like, ever seen dynamic speakers on stage at a live concert?, of course not!, the metal bands use horn speakers for many reasons, the 2 15" woofers here will provide the seperation of 2 bass drums quite well, this JBL speaker should take you where you want to go, A royal Flush so to speak of, Enjoy fellow metal head, long live Rock-n-Roll!