Top Ten, roughly in order of preference:
1st: #22 The Solution: Awesome book on so many levels, and incredible development of Rachel's character. It has so many incredible scenes: the opening with David seeming to kill off the Animorphs one by one and Rachel's frustration over failing to avenge Tobias, shouting "I'll kill you!" repeatedly as David slinks away. Then Rachel stabbing the fork in David's ear and threatening to go after his family--David later reveals this to the Animorphs and none of them come to Rachel's defense. Rachel's suspicions about Jake, Rachel comforting her sisters only to be mocked by David as she's about to shower, the situation with Saddler and Rachel's long conversation with Jake, concluding with them hugging, and then the Animorphs final strategy to defeat David. The threat of David's presence gives the whole book this climate of fear, especially as he gets closer to Rachel and is able to torment her psychologically. And I love how complex Rachel's character is in this one, how we see her deep concern for certain people, her vehement hatred of David, her introspection and her emerging inner darkness.
2nd: MM#4 Back to Before: I love Tobias's tragic storyline in this one. I feel deeply sympathetic towards him, and I like seeing how he starts to develop a sense of worth while playing pool. The Sharing offers him a completely new life, and it's interesting to feel his sense of wonder as he actually experiences kindness from strangers. KA also shows us Tobias's fatal weaknesses in a sympathetic way: his overwhelming self-pity and desire to escape his life. It's sad to see how Tobias looks up to Jake and wants to be more like him, only to feel betrayed when Jake leaves the Sharing. And later Jake pointedly tells Tom that Tobias isn't his friend. The scene when the Sharing finally turns against Tobias is one of the darkest and most tragic moments for Tobias's character throughout the series, I think. And when the Yeerk is shuffling through Tobias's memories before they die, Tobias gives a speech that's worded in a way that seems to be KA's earnest advice to anyone who is contemplating suicide.
The other storylines in this book are also incredible, and they offer a very fresh perspective of the characters we've known for so long and the Yeerk invasion. It's really unsettling seeing how the kids slowly discover the truth about the Yeerks without Elfangor explaining everything to them. Tom is a fairly scary villain since Jake is no longer aware of the menace that he is and starts making dangerous mistakes. I love how Visser One deals with Marco and Rachel chasing her, slamming the revolving door against them, attacking with a broken bottle, and summoning her thugs when she gets cornered in the alley. There's the hilarious scene with Ax in the mental hospital. And finale, we get the full-blown alien invasion of Earth, with spaceships chasing the kids through their neighborhood and the deadly battle at the mall. Awesome story, and the opening and closing scenes with Jake and the Drode are great bookends for it.
3rd: #30 The Reunion: The three way war between Visser One, Visser Three, and the Animorphs. Awesome. It's another one of those books with an intense plot, numerous highly memorable scenes, great humor, strong characterization, and interesting philosophy. Eva telling Visser One to go to hell, Visser One giving the Animorphs her email address, Marco imagining the boy learning about the Animorphs in history class, Marco resenting his friends' pity, Tobias destroying Cassie's window, the Animorphs pretending to be Andalites while Visser One makes her way to the mountain, Visser One discovering their secret, and the final battle atop the mountain with Marco finally revealing himself to his mom. Great stuff.
4th: Visser: Extremely well written. The Yeerks invade Hollywood: brilliant. Edriss 562 is utterly fascinating. She's hideously ruthless and evil, and yet, she manages to develop some human goodness. Eva even comes to the aid of her tormentor when she discovers that Edriss is also a mother. Visser Three is at his most vile, attempting to force his rival to murder her own son. The book takes a very interesting look at human nature, exploring the greatest strengths and deepest weaknesses of our species, and I love how Edriss's plan to conquer Earth is based on exploiting basic human weakness. And then there's finally the reunion between Marco and his mom, and Marco explaining the saying "Live free or die" to Edriss. But then Eva makes the heartbreaking decision to return to her captors, only to be sent off to Anati.
5th: #53 The Answer: Brilliantly juggles the themes and characters of the series. Totally epic and climactic and nerve-wracking and shocking and brutally tragic.
6th: #43 The Test: I like seeing how far Tobias has come since the beginning of the series. He's much stronger here as he struggles to come to terms with his past. I love the scene where he morphs Taylor, and how Tobias describes the experience. The insatiable hunger of the Taxxon is an interesting theme for the story. The atmosphere throughout the book is fairly unsettling, since we don't know just what Taylor is actually up to. The ending line is great: "The past is the past, Tobias. Let it go."
7th: #45 The Revelation: I love the opening scene, the happy dinner Marco's family has before absolutely everything is flipped upside-down. Marco's dad: "I have a feeling things will never be the same for us again." Marco manages to get his dream to come true, but sacrifices Nora in the process. The action sequences are extremely cool.
8th: #13 The Change: I really like the scenes between Tobias and the Ellimist, and the ending, with Tobias revealing his human form to Rachel.
9th: #7 The Stranger: I like the side that we see of Rachel in this one. Her frailties all come to the surface and she envies the grizzly's complete confidence. The future world is really cool, and I liked how Tobias briefly returned to his human form. The whole deal with the butterflies and hurricanes is neat, makes me think of the song by Muse.
10th: #8 The Alien: Another great character-driven book. Extremely hilarious and poignant at the same time. I like how it explores Ax's relationship with each of the Animorphs individually before he makes the decision to trust them and accept them as friends.
Books that barely missed making the top ten, in chronological order:
#3 The Encounter
#6 The Capture
#19 The Departure
#35 The Proposal
#49 The Diversion
#50 The Ultimate
Also considered:
#17 The Underground
#23 The Pretender
#26 The Attack
#33 The Illusion
#48 The Return
#52 The Sacrifice