For a rock band, it's the vocalist. They always need a strong frontman/woman, and unless it's Van Halen or AC/DC, you can replace the guitarist and most people wouldn't notice the difference.

For a metal band, I'm split between guitarist and drummer. Good metal is highly based on pounding, driving rhythms, a wimpy drummer makes the whole band sound wimpy no matter how badass the stuff they're playing is. However, metal that doesn't bring the riffs is usually found in the trash bin, but lead guitarists matter a lot less unless it's tech death (and I say this as a lead guitarist, unless you've got someone in the top 0.01% of guitarists we're entirely replaceable with a wide variety to pick from)

---
"I'm the straightest guy on this board. I'm so straight that I watch gay porn." - Smarkil

What's the most important part of an established rock/metal band?

[none]

[none]

---