Hey, if you're doing secondary battle morphs, don't forget Jake's rhino. I've often thought that, if I ever picked a battle morph for myself, rhino is probably what it'd be. Near-impervious hide, bad vision (which is, oddly enough, a perk, at least if you're a coward like me, but of course none of the Animorphs are), and ability to inflict serious damage just by swinging your head around? Sold.
Also, I might be wrong, but didn't Tobias use Hork-bajir as his battle morph more often than Andalite? I only recall him morphing Andalite in battle once, and Hork-bajir a couple times.
All-in-all, though, I gotta vote Rachel on this one. For her primary AND her secondary battle morphs. She could kick butt on any of the other Animorphs, either way. (That is, assuming the question is asking about brute strength. Strength isn't always the 'best' strategy. I think that was kinda the point of how the Animorphs operated, and why they all chose different battle morphs. There often wasn't a 'best' one for every situation, because they all worked best together, and played to their strengths while covering each others' weaknesses.)
Can you still add options to the poll? Collette's crocodile, Timmy's bobcat, and Kelly's buffalo are at least worth a mention as awesome battle morphs.
. . . Not to mention that random kid who decided to acquire a walrus. LOL.