What's the most important chapter (character, conflict, plot, theme and device wise) and why?

Ultimately, this question is up to each individual reader.  I don't believe a single chapter could be chosen by all readers as the most critical chapter to the book.

For me though, I choose chapter 8.  I choose chapter 8 because I see chapter 8 as a turning point to the entire book.  Previously Ralph had some control over most of the boys.  In chapter 8, the reader sees that the power has shifted much...

Ultimately, this question is up to each individual reader.  I don't believe a single chapter could be chosen by all readers as the most critical chapter to the book.

For me though, I choose chapter 8.  I choose chapter 8 because I see chapter 8 as a turning point to the entire book.  Previously Ralph had some control over most of the boys.  In chapter 8, the reader sees that the power has shifted much more toward Jack.  This is visibly apparent when Jack takes control of the conch.  

The sound of the inexpertly blown conch interrupted them. As though he were serenading the rising sun, Jack went on blowing till the shelters were astir and the hunters crept to the platform and the littluns whimpered as now they so frequently did.

Additionally the reader is shown two key pieces of information regarding the beast and the theme of evil.  Jack and his boys absolutely believe that the beast is real and must be hunted and killed.  Soon after though, the reader learns that the beast is not a real, physical beast.  Simon learns that the "beast" is not a physical being at all.  It is the potential for evil that lives in each of the boys.  Simon passes out at the realization that each boy is capable of being the beast.  

“Fancy thinking the Beast was something you could hunt and kill!” said the head. For a moment or two the forest and all the other dimly appreciated places echoed with the parody of laughter. “You knew, didn’t you? I’m part of you? Close, close, close! I’m the reason why it’s no go? Why things are what they are?”

The chapter is also important because it is the last full chapter that Simon is alive in.  Early in chapter 9 the frenzied boys savagely beat Simon to death, because they mistook him to be the beast.  With Simon gone, there are no longer any inherently good characters alive anymore.