While I don't think Rachel was psychopathic or insane I do think the other Animorphs viewed her that way, I think to a degree they'd all have had problems following her orders long term. Rachel wasn't incapable of functioning outside of war, honestly I think she could have been a "real" survivor like Cassie, she might have been bored like Marco but I don't think she would have gone off playing "Rachel the Homicidal Maniac" she was poorly handled in the books not narrated by herself, the others had this image of her that, regardless of her actions, they never really seemed interested in changing. Rachel's need to always appear ready to fight is as much for her friends' benefit as anything, being self sacraficing and fearless is a decent quality in a leader but not if it costs you your image to your troops, I think the others saw Rachel as the Riddick (yes I know Riddick came out later, it's just the best analogy I can think of right now, just roll with it) of their group, the technically evil creature they sick on the much eviler creature to bring about peace, and, like Riddick, Rachel isn't actually evil, but I don't think the other Animorphs really appreciated that, I remember finding it really hard to forgive Cassie for essentially quitting the group for fear of being like Rachel, I remember thinking "you should be so lucky, princess!" but meh.
Marco on the other hand, it's tricky to say. Marco is very much a strategist and for that I like him, I always like the strategist characters, they're the best. He shows at least a degree of command ability at several points in the books and its true that after 5 he was definately in it to win it (I know, I hate myself for not coming up with a better way to say that) but I don't know how effective he would be in the thick of a fight, how well he'd pay attention to the larger picture when he's got to keep track of where everyone is, what they're doing, and fend off the Hork-Bajir's blades, in this area I feel like Jake really excelled, possibly out of necessity, still as second in command Marco definately has his merits, assuming that he can put up the right image for it, like Rachel everyone has this way of seeing Marco that isn't wholly accurate, in the heat of battle when the king is gone will you follow the Jester into war? Few people can honestly say "yes" to that.
Overall I think the idea of Marco and Rachel working together wins out, though a good second in command could well have been Cassie. Cassie who knows everyone so well she could--if she wanted to but she rarely wanted to--manipulate them all into doing her bidding ("now bring me cheese puffs! vegitarian cheese puffs!") and probably make them think it was their idea. Would they be the super Yeerk busters they might be under Rachel or Marco's rule? No, they'd probably be a lot less agressive and attack loggers, or deer hunters, but they'd all do so with a strange sort of happiness because afterwards Cassie would touch all their sadness with her magical wand and turn it into--okay this is going downhill, but seriously, Sun-Tzu said it best "Know yourself, know your enemy, find not to fear in 100 battles, know yourself and not your enemy find victory and defeat in equal measure, know not yourself and not your enemy find defeat in every battle" Cassie knew the group better than they knew themselves (minus Rachel I think), by Sun Tzu's logic she should have at least found victory and defeat in equal measure whereas Marco knew himself but not his group really, at least not like Cassie knew them, and Rachel didn't really seem to know the group or herself . . . so yeah, my bias says Rachel and Marco together, but my unbiased head says Cassie deserves serious consideration.
