What about ambiguous kills, where it's never really made clear whether they were killed or not?
[spoiler]Such as Taylor and David?[/spoiler]
Or indirect kills? Where the Animorphs are
sort of responsible for the deaths, but not really?
[spoiler]Like the Venber following the Animorphs into the hangar and then dying, or when the Animorphs helping to reprogram Erek and then he killed a bunch of Hork-Bajir?[/spoiler]
I dunno, this sounds like a cool project, but then you get into grey areas of "well, did such-and-such really die?" or "was so-and-so really the Animorphs fault?" and I think these things become more or less impossible to categorize.
Even counting only the certain-and-direct kills, though, you can be certain that each of the Animorphs had a staggering kill count.
Jake had the most kills, hands down. As others have said, not much could beat numbers like 17,000. And don't forget all the Auxilaries, soldiers, and Hork-Bajir that he ordered to their deaths in the final battle, too.
Rachel probably comes in second. Her bear morph has certainly taken down more than her fair share of Horks and Taxxons, I'd wager.
I'd be willing to bet that Ax is third. His tail is awfully lethal, and he's probably one of the more ruthless Animorphs when it comes to killing Yeerks.
Cassie and Marco are probably about neck-and-neck for kills. On the one hand, Cassie is the pacifist. But on the other, her wolf morph is much more lethal than Marco's gorilla. Marco has a tendency to knock his enemies out, rather than kill them. Still, his ruthlessness probably accounts for enough somewhat-indirect kills to make up for that, whereas Cassie would try to spare her opponents if given the chance. So I'd guess they'd probably come out even.
And I'll bet that Tobias comes in last (unless you count rodents as kills, lol). He never seems to kill anything, except for the rare occasions where he's morphed Andalite or Hork-Bajir. Otherwise, he goes for the eyes, which is painful, sure, and plenty inconvenient during a battle, but not lethal. And since he normally fights as a hawk, he probably hasn't actually killed that many controllers.
Unless . . . oh, but here's another grey area. If Tobias blinded a controller, and then another Animorph killed them where they would not have otherwise been able to, whose kill does that count towards?