The beginning of the battle when Aldrea and Dak attack with the "monsters of the deep". It's in Esplins POV and he sees her coming up from the mist and he talks about how beautiful that image is and how he'll tell her that, when she is his host.
Also, Dak's amazing speech. I can't remember the whole thing but it ends with: "No, we're too simple for that. We're too stupid to lie and manipulate. We're too stupid to be ruthless. We're too stupid to know how to build powerful weapons designed to annihilate our enemies. Until you came, Andalite, we were too stupid to know how to kill."
Oh! and the end of the first big battle where the Hork-Bajir drop from the trees crying out "Do as HE does" and kill for the first time. It's just so epic and horrifying and I get goosebumps every time I read it.
Now I have to go reread HBC...