Pantera, Iron Maiden and Black Sabbath are true metal bands that have a great influence in the metal genres but this time I'm going to vote for Manowar. The reasons why I picked them are: the themes they approach (motorcycle riders, epic battles - steel-, metal fans etc) , the sound and their attitude. From my point of view they respect their fans like no other band (what other band sings some of it's songs also in german, french, italian, bulgarian, greek roumanian, portugese and who knows how many other languages). Manowar is a very good example that there can be metal and not noise; there can be soul and melody in a metal song.
http://www.youtube.com/watch?v=BLSVEBBb5Z4

http://www.youtube.com/watch?v=s2bnKeUvKG0

Iron Madien was one of the classic Metal bands

I have to say that Black Sabbath is the most metal band I know but there have been attempts to reach their hardness.. which is basically impossible as the scenario is relative.

Now, relatively speaking I think there are a lot of "hard" bands and "heavy" music but none that incorporate everything so well as the band A**l C**t, from the name, to the music, to the lyrics, to the pure "F*** everything I am going to crash in a mass murder suicide burnout that kills all of society and everything we and you know as normal" type of pure rageful "hard" rebellion .. a necessary ingredient in all modern metal.

Warning .. well just "warning" but you asked

A**l C**t

Well the question should be what is the most "brutal" metal band.

Its Decide, they may not be the fastest or the heaviest band around but they were pretty innovative for their time. Also in the metalocolypse episode where they show Nathan Explosion's childhood, a man drives his truck into Nathan's school and before hand screams "Im a Lunatic of Gods Creation"

Dont quote me on this but I believe the singer of deicide has done voiceovers for the show.

Deicide- "Lunatic of God's Creation"
http://www.youtube.com/watch?v=MPvRBiQY6lo

Why this is so metal to me is it sounds like its full of hate, and all the members are full of hate. I dont agree with really any of their politics, in their music or personal opinions, but i think thats the point. They are angry and they want you to be. Also the singer is crazy and burned a cross into his forehead 12 or 13 times and you can still see it. If god was going to strike anyone down it would be this band.

Hope you don't mind an old timer's point of view. Growing up in the late 60's and 70's, I cut my "metal" teeth on Black Sabbath's great "War Pigs", surely a foreshadowing of greater things to come! I never considered bands like AC/DC, Judas Priest, or Kiss to be metal, although I have heard all of them described as such. Too much dependence on melodic and even harmonic vocals, in my opinion. You could probably make a metal band out of any of them if you turned the drums up and the bass, played a few more power chords, threw in a few more screams and growls, and just basically quit being so NICE. The big four, of course, that every right thinking music fan knows, is Metallica, Megadeth, Slayer, and Anthrax. All four have some nice examples of metal in their repertoire, but I hesitate to call them the MOST metal band. Of course we run into the complication of sub-genres now, like speed metal, death metal, thrash, etc. And of course those crazy skaters have got their own tunes all to themselves, some of which rock pretty hard (or should I say "metal" pretty hard?). My personal favorite is a thrash band called Suicidal Tendencies, but they are a long long way from the MOST metal band. Of the mainstream bands, I'd have to choose Pantera. Hard driving bass and guitar power chords, killer drums and angry angst-filled vocals. I love their sound.

I suspect that there are some underground metal fans who take this uber-serious and would claim Pantera is a sell out and tell me only their favorite band, Deathscreams Daily, is REAL metal, and for them, that may be reality, although I doubt it, since I made that name up. in case anyone is concerned, the genre is alive and well, plenty of fans, plenty of up and coming bands. Talk about a cathartic revolution, if you can't work out some of your angst with this music, the rest of us may be in trouble.