Ax's tail length always bothered me, as other people have said here as well. It didn't seem long enough to be able to reach above his head to attack anyone. I felt that Visser got that right the best.
Andalite Chronicles is probably my favourite special edition cover. Everything including the inside cover with the Animorphs looking down at him were really well done. Other covers that I also really liked: #2, 3, 19, 39, 43, MM#3, MM#4, Hork-Bajir, and Ellimist.
As for the worst cover, I didn't like #54 all that much at all. It didn't seem like an Animorphs cover. Also, the models in the last book were changed AGAIN. Rachel especially looked different from her previous book #48, and the one before that in #42, then #37, and then #32! If they really had to change the cover models, then I really wish that they had stuck with one and signed them up on a contract or something so that it'd all at least look consistent for all the future books. Instead, we were left guessing who these people were for #54.
Cassie obviously looked the same because it was the same person. Tobias and Marco's model were the same from MM#4. Ax was a cardboard cutout, so he didn't change. So it was really Rachel and Jake that bothered me the most here. Jake especially didn't make any sense. The model they used for #54 was the same model from #47, however, they used another model in #53 that looked like the older original Jake, yet in the next book #54, they revert back to the guy from #47. Ugh.
All this switching around was obviously done because of scheduling conflicts, but I still hated it. It's not only that, but the fact that the Jake in #54 looks like he's 10 years old, and younger than everyone else on the cover. Can you honestly look at him and think of him as the leader?