Up to around #30, the series is quite good. Once some of the ghost writers come in it goes either way. So my shortlist would be:
#31 is ....meh. It has some good parts, but otherwise I'm not to interested in it.
#36 is .... It's okay. I love the idea of an "Atlantis" type place, but in the Animorphs Universe?
#37 .... No comment. I mean it's a good plot, but badly and hastily written. My LEAST favourite book.
#39 .... Eh ....
After 40, there are some awesome stories.
But ....
42 .... It's sort of been overdone, storywise. But it's okay.
48 .... Oh man, I cannot stand this book. I think I might have only read it that one time, because I'm looking at it now and it's in pristine condition.
The last few books were good, had few problems with them. Although #54 was a little disturbing. I loved all of the Chronicles (Ellimist is odd, but has some interesting imagery) and Megamorphs.
Alternamorphs I never even bothered with.