I'd probably say I preferred #28 (The Experiment) and #35 (The Proposal). Partly because I prefer Marco and Ax narrations to Cassie and Rachel ones, but mostly I just preferred the plot lines of those books.
#28 was funny, but still had some serious moral and ethical issue, and I found it interesting to see Ax's POV of how animals were treated on the Andalite homeworld. As a species, Andalites have no need for animals as meat or clothing, and they apparently find the idea of animal experimentation barbaric.
As for #35, it was interesting to see how intense stress affected the morphing ability. I sympathised with Marco as my parents are both getting remarried, although neither of mine have alien slugs in their brains (as far as I know). Also, I liked the part near the end when Marco mentions his new Star Trek idea (it sounded really cool!)
So why these 4 books exactly?