Sorry to bump, but I'm shocked that you guys seem to think Rachel had a relatively easy time with it. I think, her death notwithstanding, she was the most emotionally broken of any of the Animorphs--save maybe Jake. Rachel started out the series as a strong female character who wanted something more than what the world wanted out of her. She found that through the war, and at first she really liked it. But she completely lost herself to that and, unlike all the others (save Jake), wasn't ever really given the chance to deal with the terror and emotions she put herself through because she felt that she was strong enough to deal with it. So she had to keep getting stronger and stronger, until she lost herself. There are two ways to completely lose yourself to the war, I guess--to become too weak to fight it, or to give yourself up completely to the fight and rush.
Rachel had to give herself up completely. There was nothing in her salvageable after the war. She could NOT have coped or found a way to integrate herself into normal society. At best, she could be a ruthless military spy of some sort.
Jake is fairly similar in regards to being unable to find his place in the world afterward.
However, I believe that Tobias would have been able to do this--especially with Loren back--until Rachel died. Cassie and Marco ("the only real survivors") were obviously able to, though not perfectly. Ax, well, I don't get his resolution or situation. So I'm going to say he didn't have as hard of a time AFTERWARDS as DURING and that was due mostly to his separation from his culture.
So speaking from an after-the-war standpoint, looking at who would be most capable of integrating themselves back into society:
Cassie > Ax > Marco > Tobias > Rachel = Jake
Easiest to Hardest, sorry it's in the opposite order!
During the war, life sucked for all of them, but I could see it sucking a *little* less for Rachel because loving it was the only way she could deal with it and Cassie because her justification seemed to help her cope. Tobias had that bird thing, but he also seemed to be more happy with himself than before the war because he finally had a makeshift family of sorts ("go to your friends, tobias. they are your family now.") and belonged to something. and as MM4 shows us, Tobias had a better life in the war than he would have had without it. Jake's position sucked during the war as did Ax's and Marco's. So I really don't want to say that they can be RANKED for their sufferings during it...they all seem pretty damn equal.