@TheGreatCthulhu can we have the answer yet?
What I think are the most important?
Easy. Drums and bass.
For several reasons.
I know most people will tend to focus on the guitars and the vocals, while very important, they aren't the foundation of a composition.
Without rhythm, we have no interesting music, and it's a drummer's job to set the rhythm, set the tempo, and keep everyone on time. This is why for drummers, especially in metal, timing is important, as is hit strength. A lot of people will play drums with a light touch, and that's the wrong way to go with metal drums.
I like it when drummers are in rock-solid time and are just pounding the crap out of their kit. It sounds the best, and gives the drums a complex, rich timbre.
It's also why in metal drummers always play steady rimshots on the snare, where they're hitting the head of the drum and the rim of the drum at the same time. This results in an explosive, tight snare sound that will help the engineer in mixing later. You just gotta have the drummer to stop hitting the drums with his purse!
Bass I believe is the most underrated instrument in heavy metal. I can't fathom why as bass adds girth to the drums, and a gigantic bottom end to the guitars, making the band sound huge.
A good bass tone, played by a great bass player is arguably the most important role there is.
Which also pisses me off, because a lot of people starting bands have this problem. A group of friends decides to start a band, what ends up happening is this:
The
talented guys wind up on guitars or drums, the guy that can sing or growl, winds up on vocals, and the awful guitar player winds up on bass.
And it's those jerks that don't give the instrument the respect and dignity it deserves. They don't maintain their instruments, leading to strings that are 10 years old sounding like crap, basses that I'm surprised still are working, and routinely can't play their parts.
If one aims to be a kickass heavy metal bass guitar player, one ought to listen to the likes of Steve Harris from Iron Maiden, Lemmy Kilmister from Motorhead (RIP), Al Cisneros from Sleep, Cliff Burton from old-school Metallica (RIP, when 'Tallica used to be awesome), David Ellefson from Megadeth, Justin Chancellor from Tool, Ian Hill from Judas Priest, and other many great and talented bass players to get inspired.
All of these bass players have several things in common. They're always in key, on time, they drive the rhythm of the song, they add girth to the drums, making the drummer sound better, and they add girth to the guitars, making the guitarists sound better.
In other words, if you don't have a rock solid foundation, the other things that are building on top of that are going to crumble.