I have two answers here.
1. I would love to morph the duck, because of its restlessness in flying. I love the way Applegate describes the flying sensations; it really drove me to envy the characters because they were experiencing it.
2. Because I am constantly reminded on how the Animorphs (AM) generated many nightmares from morphing this insect, I would count it as my favorite AM's reaction to morph. It's the ant. I like how morphing it can be equated to being infested by a Yeerk - same feeling of helplessness and having mo freedom. I can't choose what's worse - being a prisoner of a system (work-kill-die) or having your actions controlled by an evil parasite. I love how that chapter made me think.