what would be the easiest to difficult roles in your standard metal band.

Entirely depends on the band. Pretty impossible to say.

Also don't underestimate vocals. Performing at the level these guys do with high energy while screaming/singing LOUD for an hour or so is HARD. That shit requires endurance.

I would say I am personally more impressed by an incredible drummer than anything else.
"Standard" metal band is kind of pointless without a working definition. Do you mean in terms of instrumentation?

Also different subgenres of metal require different things out of different instruments. The keyboard player might be the most demanding role from a technical point in funeral doom for example. The bass might be the most demanding in any sort of funk inspired metal or any band with extensive slap bass, such as Suicidal Tendencies. Similarly in many styles the bass is expected to follow the guitar verbatim (albeit an octave down). Say what you will about goofy tech metal bands, but doing the same extensive tremolo, tapping, and sweep stuffs on bass as on guitar is not something to joke about.

Ultimately not a good question because a) it's vague, b) it has too many variables, and c) is too subjective.

I'd say the hardest role is guitar though because guitar players tend to lack good judgement and tend to serve themselves rather than the song more often than other musicians. Sometimes the best thing to learn when having a conversation is know when to shut your mouth. The same principle applies to music.
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Actually, I change my answer to drums because setting up for them is definitely the hardest and in general it's more fun to set up your amp and pedal board and then drink or eat or chat with females than it is to help your drummer carry or set up their shit.
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