I have always been an avid hater of the ending of Animorphs, for many of the same reasons you guys have stated. But on a closer look, I think thats the point.
We aren't supposed to like how Animorphs ends. It ends with tragedy, loss, depression, and hey, guess what, another war to top it off! Yippee!! It isn't fair, it isn't right and it certainly isn't a video game victory. And that was the point.
I don't like the ending. It pains me to see the characters I loved and followed for six years reduced to realistic war wounds and deaths. I don't like it, but I can respect it. Because it really is a realistic representation of what war is about. I think a "perfect ending" would have cheapened the series.
Also, I'm not sure what everyone means about Rachel being one-dimensional (or any of them really). I thought Rachel was extremely one-dimensional in a lot of the ghost-written books, but in the last book we finally got "our" Rachel back for like...three chapters.
Anyways, as I said. I don't like the ending and I never will, but I can respect it.
I think the one change I would suggest would be to make it longer, like MM or Chronicles length, so it seemed less rushed.