Voice in writing is very hard to describe, except to point to examples that illustrate it. It’s also something that develops over time–one of the reasons that many of us have to write a few unpublishable things before we reach that magical level. Sometimes, it’s the thing that allows you to recognize the work of one author over another.
But voice is more than just authorial voice. There’s also the voice of a story (a fable should sound different than an adventure story) and the voices of characters.
There’s no doubt that voice is important, especially in young adult stories, but it’s not more important than the story. In my opinion, nothing is. And the voice has to match the story.
A story told in first person is almost always going to have a stronger voice than one told in third person. In fact, in my opinion, it should, since in essence it purports to be the character telling the story. Done right, it should sound like that character. I’ve seen several novels that used first person without making it sound substantially different than a narrator’s voice.
In contrast, a story told in third person is mainly going to be in a narrator’s voice (and probably a lot closer to the author’s voice). Even in third person, though, the closer the story is to the point-of-view character, the stronger the voice.
But, that doesn’t mean that in order to have stronger voice, every story should be told in first person or even close limited third person. That depends on the demands of the story.
For example, stories that have two (or more) point of view characters can be difficult to pull off in first person. First, it’s confusing to read if both characters are written in first person. Who “I” is changes from chapter to chapter or even from scene to scene. Second, two first-person point of view characters probably ought to sound different from each other–which is probably pretty difficult to pull off. I say that, because I haven’t yet seen an example that really did pull it off.
I’ve written a couple of short stories in first person.–one because the subject just seemed to demand that closeness to the character and the other because I just started hearing the story in first person in my head and decided not to fight it. I haven’t–yet–written a whole novel in first person. I probably will some day, when a story tells me that’s what it needs.
In the meantime, I think of all the great stories I would have missed if I’d demanded that everything I read had the kind of voice found in a first-person narrative.