Hmm. I've been giving this a lot of thought, and it can go a bunch of different ways, depending on what you mean by the original question. As such, I'm going to give my interpretation. Apologies in advance for the wall-o-text. Like I said, I've been giving this a lot of thought. That's why I haven't posted before now.
If you mean, "Who disliked/was changed by the war itself?" then that's easy enough to order. Cassie > Jake > Marco = Rachel > Ax > Tobias.
Tobias was the only animorph who never had any doubts about what they were doing. Sure, he had doubts about being a hawk, and he felt fear before going into battles, but he never once (to the best of my recollection) thought about doing anything besides fighting Yeerks. And I think I know why that is, too. More so even than Rachel, the war was actually a blessing for Tobias. Why? Because it gave him a way out. A way out of his life, a way out of the pointlessness of his own current existance. I tend think of him as an escapist. When he's stuck in a situation that he doesn't like, he'll try to escape from it, any way he can. Even if that means becoming a hawk and fighting a war against an alien race. For him, that was actually a step up from the life he'd had up till that point. Because at least with the war, his life actually meant something. So I think he actually had the easiest time dealing with the war itself. Now, his identity issues with being a hawk complicated matters a bit, but as far as fighting Yeerks? He was pretty darned straightforward.
Ax comes in second on my list, because he had time to mentally prepare himself for war beforehand. He'd joined the military out of his own free will, as opposed to the other Animorphs who were just thrust into the war. And it shows, too. Aside from a few dilemmas about which race he's actually fighting for, he never had many problems with what they were doing. And having the noble motivator of "honor" helped him out a lot, too, because he was able to fight for something greater than himself.
Rachel and Marco are about equal. They've both had moral qualms about war, and doubt themselves a lot more than either Ax or Tobias ever did. On the one hand, there's Rachel, who, on the surface, looks like she's enjoying the war. But it changes her, and for the worse, I think. I disagree with everyone who has thus far said that she would be able to integrate back into society and be normal again. Maybe, maybe she could be with some kind of military operations, and cope that way, but otherwise, no. She is too far gone by the end of the series to ever come back. And she knows it, too. That might be part of why she pretends to like it so much, because she eventually can't even admit to herself how much it scares her. It's a vicious cycle that pulls her in until she can't get out.
Then there's Marco. Like Rachel, he appears to be coping well, on the surface. But, as we saw in book #35, things really do get to him. And he develops as much of a 'dark side' as Rachel does, it's just that his is more subtle, and, in a way, almost darker. Rachel is straightforward with hers, diving right into battle yelling "Let's do it!" But Marco becomes a cunning and ruthless tactician who would shove his own mother off of a cliff if it might help them win the war. You cannot tell me that a person could come out of a decision like that unchanged. And I don't think he did. I just think he took all that doubt and shoved it deep beneath his funny/ruthless exterior, until everybody else thought he was coping fine, when really he was broken inside.
Jake, of course, has the unique burden of being the leader, and it weighs on him, especially at the end. All the decisions he's had to make were decisions that nobody can make, and it wrecked him. He actually dealt with it amazingly well up until the end, but he just hit a point when he couldn't take it any more, and fell apart.
Cassie. Cassie, Cassie, Cassie. The moralizing hypocrite. Of course she couldn't deal with the reality of war. How can you possibly keep the idea that "all life is sacred" while killing innocent beings? Actually, out of all the Animorphs, her character struck me as the least realistic. The war never even seemed to change her at all. She clung so rigidly to her morals, at the expense of, oh, I dunno, ALL RATIONAL THOUGHT, that it seemed almost contrived. But her morality may have been a coping mechanism, too. She was so unable to cope with war that she just stuck steadfastly to the very other end of the spectrum, and refused to make so much as ONE sacrifice, even to save millions. That kind of attitude is unhealthy. It's as sure a sign as any of being flat-out unable to cope with reality. Which is why I was always stunned at Cassie's quick recovery after the war. Of all the Animorphs, if any of them ought to have been left broken by the war, it should have been Cassie. My reasoning for that is that she's STILL in denial about everything, even three years later, and simply does not realize just how damaged she is.
To contribute a word or two to the current discussion:
Yes, the Animorphs could have had breakdowns during the war, and yes, that might have been more realistic. But how fun would that have been to read? I don't know about you, but I'd rather read about kids fighting a war against aliens, than about kids who go nutso and then can't cope with reality and sit around all day making raspberries at each other in their padded rooms while the aliens take over.
