Well I am going to be different and vote one of my favorite Cassie moments had to be when she went humpback whale to get rid of Visser Three's "pet" at the end of Megamorphs #1.
When I first saw the thread title, I actually sort of assumed this was what it was. Greatest hits, and all. Granted that I'm no fan of Cassie's (have to agree with the general sentiment by pointing out she's my least favorite character, and that includes Taylor and David by the way), but come on, there are
plenty of threads already out there for talking about her lowest points. Let's change the tune a bit, eh? QIfry has the right idea, I think.
My all-time favorite Cassie-moment was in #50, when she was furiously explaining to her parents that this was a war, and people die. This is
Cassie, the most innocent by far of the Animorphs, explaining to her
parents, which are supposed to be the role models and authority figures of any kid her age, the gristly and horrifying realities of war. That was a beautiful scene, chock full of lovely character development. That (ought to have) really brought the war home for her.
Shame that she lost all that character development even before the end of the book. But still, it was nice while it lasted.