My order:
Marco & Cassie
Rachel
Ax
Jake & Tobias
I loved Marco because he was funny, but also very smart. He complained a lot, but he was always there and he was loyal.
Cassie was very down-to-earth and she always stayed real. People can think she thought too much about morals, but in my opinion, that was a good thing. I realise that when you're fighting a war, you can't feel sorry for everyone and think too much about what is right and what is wrong, but at the same time, you need someone like that.
If Cassie had fought the war herself, Earth would've been invaded for sure. But if any of the others had fought the war themselves, the same thing would've happened.
What made them strong was all of them together. They were all so different and while Marco and Rachel didn't feel like there was any point discussing morals as long as they won, Cassie thought about these things a little too much. But they never didn't do something just because she didn't like it, and they never just did something because Marco and/or Rachel or someone DID like it, or said it was necessary anyway.
I can understand why people get tired of Cassie, but the series needed her. The Animorphs needed her. She was the voice of sanity, so to speak. The others all often said that Cassie was different, that she could see things on a different level and not just think about what was there at the moment. And I think it was hard for her, being the only one like that.
Of course she thought so much about morals, because she was the only one doing it. Okay, that's not true - obviously the others also wondered whether what they were doing at the moment was really okay or not, at least Jake and Tobias. But not the same way.
Also, it was simply her personality, which of course is either very likeable or unlikeable
Also, the whole thing about Cassie becoming the author and ghost writers' "pet" and all these things happening to her:
From the very beginning we had it explained to us that Cassie was special. Cassie wasn't like the others, and not just personality-wise. She was special.
Maybe these things were all planned from the beginning by the author/ghost writers. Maybe there were always special things meant to happen to Cassie. We always knew she was special, so is it really that strange that these things happened (or didn't happen) to her?
I also want to say that I have only read books 1-19 + The Andalite Chronicles + Megamorphs 1 & 4. There's a lot of stuff I've read a little about, but not yet read in the actual series, so there's a lot of stuff I don't know yet.
I know that Cassie later somehow betrays them, but I also know that it then turns out that she didn't actually betray them, only the others couldn't understand that at the moment. Also something about Cassie being different and seeing things on a deeper level.
Sorry, I haven't read about that yet!