(note: this is not your most/least favorite)
which book in the whole series, chronicles included, do you feel is the MOST and least important to the storyline?
for more important book, i'd have to say the #1, the invasion, but the andalite/hork-bajir chronicles were also quite important.
the least important book would definitely be #48, the return. complete waste of a book, it was a little touching at the end but was kind of pointless.
i might go over the 'most important' choice again, because i'm sure there's a good second candidate, but number one was definitely needed.