This is a really tough decision. They all have their strengths and weaknesses, but I'll have to go with Tobias, followed by Rachel, Marco, Jake, Ax, and finally Cassie.
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Tobias is by far my favorite. I've had a terrific childhood, but I find him oddly relatable. All of his books felt very well written to me. It just bothers me how in the end, Applegate screwed him. He never got to have the life he deserved.
Rachel's reckless and violent nature intrigues me, but I feel like more should've been done about her character. She felt kind of underdeveloped in the end. They really could've done more with her. It seems odd then that I would have her be my second favorite. It's just that there was so much potential in her that I couldn't put her any lower. I almost want to say that I relate to her as well as Tobias, but I don't. There are only small bits of her personality that I relate to, but I still
What's not to love about Marco? He's sarcastic, logical, cautious, and would've made a much better leader than Jake were he able to make rapid-fire decisions under pressure easier. The ongoing story about Marco's mother was great, too. I simply liked Rachel a bit better.
Jake actually kind of bored me at times. He's just so average that he gets kind of bland. However, he does have some really great moments with Crayak and the Ellimist as well as some interesting moments with internal struggles about leadership. And who can forget possession by Yeerk!
Ax's comic relief made him my favorite when I was younger, but now he really isn't that interesting of a character.
Cassie has always been a huge annoyance for me. Her need to go over every decision with a moralistic fine-tooth comb just irritated the piss out of me. One good thing about the last several books was Cassie's ability to finally see that they are in a freaking war and you can't win by planting flowers and singing songs at the enemy. This change in character is slight and isn't really noticeable until she argues with her parents about certain war tactics.
Interesting enough, some of my favorite books are Cassie books. The Aftran books were brilliantly done and Cassie's hyper-idealistic attitude actually has merit here. Of course, she then has to ruin my appreciation of her in these books with her terrible decisions and attitude in others...