The One Tree gets my vote for worst book in the chronicles. But I often compare it to Empire Strikes Back...it's the middle book, the one with character growth and plot development, but is doomed to be irrelevant by it's mere existence.

I read the first chronicles in 5th grade, after LoTR, which would've made me 9 or 10. Didn't understand much of it I suppose, until the 2nd or 3rd reading.