I don't think the North would have been determined enough to secede. After all, they'd gone along with many administrations that'd given the South everything they wanted. Plus, abolitionism wasn't anywhere near as popular as defending slavery was in the South, and no one except a few radical abolitionists like Garrison had ever talked of secession in the past forty years. So, the question is who would aggravate the South enough to secede.
Breckenridge is right out; he'd give them virtually whatever they wanted.
Bell is out as well; he'd represent a continuation of existing policy. The South might not like that, but it wouldn't represent anything the radical fire-eaters could spin into justifying secession.
Douglas is less certain. He favored leaving slavery alone permanently, and he was neutral on its expansion. Still, the South abhorred him after he'd abandoned them over Kansas.
Lincoln is certainly in, as per OTL.
Therefore, Lincoln is the worst for the Union in the short term because he is the best for humanity.