In a heavy metal steel-cage death match involving every heavy metal band that ever was, Iron Maiden would win.
The following essay is my explanation of why I believe Iron Maiden is the best metal band of all time, better even than Black Sabbath and Judas Priest. Against some stiff competition, Maiden would emerge from this epic fight of metal heavyweights bloodied and bruised, but victorious, standing atop the heap as the most consistent and best metal band of all time.
Iron Maiden is my top pick not only for the soaring heights of their peak, classic albums (which include Number of the Beast, Piece of Mind, Powerslave, Somewhere in Time, and Seventh Son of a Seventh Son), or for their groundbreaking punk/metal fusion early albums with then-vocalist Paul DiAnno (Iron Maiden, Killers), but for the amazing music they're still putting out today, slightly lesser known but terrific titles like Brave New World, Dance of Death, and A Matter of Life and Death. Greatness is measured in many ways, and one yardstick is longevity. All three of these latter critically acclaimed albums came out after 2000, in Maiden's fourth decade. Maiden is still as creatively fresh and vital as they were in the mid 80's.
In this regard, Maiden has it all over Black Sabbath, whose classic self-titled debut, Black Sabbath, came out in 1970, but whose last great album was 1982's Mob Rules. A great 13-year run, but not nearly as good as Maiden. Likewise, Judas Priest has put out exactly one good album (2005's Angel of Retribution) in the last 17 years. Harsh, but true.
...close, but no cigar for Sabbath, Priest
Black Sabbath is the popular choice in most "best of" heavy metal polls, but I think a huge part of the reason is simply because Sabbath was first on the scene. Their self-titled debut is generally regarded as the first pure metal album, and as pioneers they deserve incredible respect and suitable props. And I'll also admit that Tony Iommi might be the best metal guitarist ever--not necessarily for technical ability, but for his unique sound and amazing riffs.
No one can doubt that classic sabbath albums Black Sabbath, Paranoid, Master of Reality, and Sabotage are the very essence of metal, or that the very names of Black Sabbath/Ozzy Osbourne aren't inextricably linked to the public perception of heavy metal. When you say 'heavy metal,' the mind immediately conjures up the classic Sabbath lineup, pounding out the headbanging "Paranoid."
But Sabbath hasn't put out a decent album in 26 years, and there's too much water under the bridge now. They hit with a hammer impact that will last forever, but ultimately, their window of greatness closed too quickly and their catalog is a bit too thin. Plus, to be honest, Ozzy has tarnished his legend with his recent buffoonery on and off-stage, and even at his peak was nowhere near the talent of a Bruce Dickinson or Rob Halford. So while Black Sabbath started it all and set the ball in motion, they've since been surpassed.
Judas Priest is the other true contender to the throne. In fact, like Maiden I think Priest is also better than Sabbath. Like Sabbath, Priest pre-dates Maiden, arriving on the scene well before Steve Harris and co. with 1974's Rocka Rolla. While Maiden was still kicking around in clubs, Priest laid the foundations of the genre with some seminal albums, including 1976's Sad Wings of Destiny, 1977's Sin After Sin, and 1978's Stained Class. Although these albums aren't among my favorites, they contain some great songs.
Arguably, guitarists K.K. Downing and Glenn Tipton are better than classic Maiden counterparts Dave Murray and Adrian Smith. And you can even argue that Rob Halford is (or at least was) a better singer than Bruce Dickinson. But I'll leave the best metal vocalist of all-time discussion for another day. Priest has also written some of all-time metal classics like You've Got Another Thing Comin', Breaking the Law, The Sentinel, The Green Manalishi, Beyond the Realms of Death, and much, much more.
But Priest has some flaws that drop it a notch below Maiden. Most notable is a very uneven discography, noteworthy for its soaring heights of greatness interspersed with some real lows. For every Screaming for Vengeance and Defenders of the Faith there's a Point of Entry and a Ram it Down. Then there's Turbo, which (although I like this album), is pretty much regarded as a sell-out. I love Priest, they're an all-time great, but selling out is something Maiden never did.
Priest put out its last great album in 1990 (Painkiller, one of their best) but then spent 15 years in the basement, releasing a few forgettable albums with an unremarkable singer (Tom "Ripper" Owens). When Rob Halford rejoined the band Priest pulled itself together with a very respectable release, 2005's Angel of Retribution. But while good, it's not in the same class as any of Maiden's albums after the reunion with Bruce.
Thus, Maiden wins out over Black Sabbath and Judas Priest in consistency and longevity.
But more than years of service, Maiden wins out for its a unique combination of musical ability, showmanship, songwriting talent, and great marketing. Musically, Steve Harris is a terrific bassist and Dave Murray and Adrian Smith are vastly underrated guitar players with a number of great solos to their credit. I don't know a damned thing about drumming (other than Neil Peart is God) so I can't comment on Nicko McBrain. But of course, Bruce Dickinson is in anyone's list of great vocalists and is arguably the best ever.
Maiden can write fast, fist-pumping songs (Aces High, The Trooper), sing-along concert hits (Run to the Hills, Fear of the Dark, Two Minutes to Midnight), soaring epic tracks (Revelations, Hallowed Be Thy Name, Ancient Mariner, Paschendale), and everything in between. They have metal's most recognizable mascot (Eddie) and classic album covers and artwork. Dickinson is an absolute marvel on stage, even at 50, and their stage shows/props are great.
Of course, Maiden isn't perfect. 1990's No Prayer for the Dying marked the start of a downhill slide that hit bottom with the Blaze Bayley-fronted albums The X Factor and Virtual XI. But they've rebounded hugely this decade with new great music and they continue to sell out arenas worldwide as I type. What else need I say? Maiden is the best.
...and the honorable mentions
In my opinion no other metal band can even be mentioned in the same breath as The Big Three. But here are some other honorable mentions, veterans of the metal wars that garner a lot of respect (or at least did at one point in their career):
Metallica. This band was serious contender for six great years. Seriously, between Kill 'em All, Ride the Lightning, Master of Puppers, and And Justice for All, the metal sceptre was within their grasp. But sorry, four great albums--and they were great--ain't gonna cut it in this contest. The black album was the start of the downhill slope, and it's been shit ever since. A major sell-out and a major disappointment, Metallica is a study in how the mighty can fall hard.
Dio/Ronnie James Dio. Dio is awesome, and was responsible for (in my opinion) Sabbath's best album, the post-Ozzy Heaven and Hell. Dio went on to a fine solo career with albums like the immortal Holy Diver, but his is also a career marked with a lot of mediocrity.
Blind Guardian. Another awesome band, Blind Guardian is the reigning king of fantasy-based power metal (beating out Manowar in this regard, IMO). If they arrived on the scene 10 years earlier they'd be in the running.
Pantera. This great thrash band with a respectable career, but a dozen years (1990's Cowboys from Hell through 2000's Reinventing the Steel) isn't long enough to put them anywhere near the running. Arguably they held the throne for one year with 1992's Vulgar Display of Power, but they haven't done a damned thing since Reinventing the Steel.
Megadeth. Another very good metal band, Megadeth flirted with greatness circa 1985-90 with three very good albums (Peace Sells...But Who's Buying?, So Far, So Good, So What, and Rust in Peace). But overall, a great light-heavyweight that lacks the clout of the big boys.
Anthrax. A lesser version of Megadeth. Scott Ian would be the first to admit that Anthrax, despite fielding a couple great thrash albums (Among the Living, State of Euphoria), is not as talented nor as influential as the other bands on this list.
Slayer. Slayer is a great metal band, with some all-time classics, and earn points for their longevity (they're still putting out respectable albums). Reign in Blood, South of Heaven, and Seasons in the Abyss should be in any metal fan's collection. But Tom Araya is not in the same ballpark as Dickinson, Halford, Dio, and other greats. Slayer is limited by the restrictions of the thrash/speed metal genres and Araya's limited vocal ability.
Manowar. This lovable, sword-wielding bunch of maniacs may be the self-proclaimed metal kings, and have some great albums and kick-ass songs to lend support to that lofty title, but they are also rather silly.
Queensryche. This band's path parallels that of Metallica to an eerie degree. Queensryche is (or was) an incredibly talented band whose premier album (Operation Mindcrime) should make any self-respecting metal fan's top 10 list of favorite albums. They also have some other classic material (Rage for Order, The Warning) that, up until 1990 or so, put them on or near the same plane as the other metal greats mentioned above. Then came The Crash, and I'll be kind and leave it at that.
If you've gotten this far and are wondering where bands like Led Zeppelin, KISS, and AC/DC are, you best stop looking and do a little more research. Folks, let's set one thing straight--KISS, AC/DC, and their ilk are hard rock, not metal. It's not that I don't like KISS and AC/DC--I love them, in fact, but metal they ain't. That goes for Deep Purple too.
And sorry MTV, you don't know shit about metal and you never will. I mean, look at this heinous list--Maiden fourth, behind Metallica? That's about all you need to know about the veracity of MTV as an authority on metal (or any form of music, for that matter).