I think the obvious answer is guitar. However you can produce rock music with out one.
For example Cop shoot cop, have two bass guitars and they create a really heavy complex rock sound.

so while guitar is the most dominant instrument in most rock bands it is not essential.

I think the drums are essential.....although now that I think of it I have seen a couple of rock bands who are a two piece and they use a drum machine......hmmmmmmm.

I guess there is no essential instruments when forming a rock band. The guitar is the instrument which stands out and which usually give rock its sound. So my vote is with the guitar, which is different to how I was expecting to answer the question when I first started writting.

i think it depends on how one interprets the question...

is it a matter of what instrument is most focal within a typical rock band?

or

is it a matter of what instrument distinguishes a band as 'rock' as opposed to another musical style?

Yes...heated is the right word! Anyway, the most important intrument varies with the music genres. If you are talking about rock (because this is in the rock section) I'd say the guitar. If you are talking about music overall, I agree that the voice is the most important instrument.

Every instrument has brought out significance into the world.

There is no such thing as the most "important".

but I know what you mean, and I suppose it's the human vocal cords (think about it).

Im gonna say Drums are the most important..... followed by the good ol' six string and some good vocals

From a compositional standpoint and performance standpoint, it's a dead heat between the human voice and the drums. Even rock with little to no guitar typically still has both.

The human voice and the drums are the two constants throughout all of music history and all cultures. Without a great singer and a great drummer, a band will never truly sound great.

And in the end, it may be drums that takes all. A good drummer will always make a band sound better, but if there's a bad drummer, it doesn't matter how good everyone else is-- they will sound less than stellar without a solid foundation.

Hmmm... it's still a dead heat with vocals though. I think when most people listen to music, they generally "hear" the vocals and "feel" the drums-- especially in live music.

I listen to so many bands without vocals it's not even funny

so no, vocals are not most important


PS: what's with all the super old thread bumps lately?

Drums I'd say. This is a pointless discussion when it all boils down to it..