I finished "What An Animal" over the weekend. Even with 12 books instead of 6 it was a quick challenge as they were short books ;) I'll continue reading the series though, so who knows - I may make it through all 64 books before the challenge is out :D
So far my two favourites were #13 The Change and #19 The Departure as I found those the two most poignant and thought-provoking so far. Yes, I know they're children's books, and crappy ones at that, but still.
The Change showed the Hork Bajir(sp?) for what they are when not infested with Yeerks, and gave Tobias another chance.
The Departure finally touched upon the moral dilemma of the Animorphs and the fact that not all Yeerks are intently evil.
Good books both of them.