What methods and leadership were most effective of the abolitionist movement?

I would argue that John Brown's raid on Harper's Ferry in Virginia was extremely effective in inflaming the sectarian tensions that led to the Civil War. The issue of slavery more than likely would not have been resolved without the war.
Brown led his raid on the night of October 16, 1859 with twenty-two men, six of whom were prominent abolitionists. By the end of the fight that broke out between Brown's "army" and the US Marines who were dispatched to the site, ten of Brown's men were killed, including two of his sons. Brown was later tried for treason and executed. Before his execution, he passed a note to his guard that read, "I, John Brown, am now quite certain that the crimes of this guilty land will never be purged away but with blood."
Brown was no stranger to guerrilla warfare. Earlier in the 1850s, he traveled to Kansas with five of his sons to stop the development of slavery in the new territory. Slavery was permitted in neighboring Missouri and there was a desire to expand it westward. Brown lost another son in the battle now referred to as "Bleeding Kansas."
Brown remains a controversial figure. Some historians laud him as a radical hero, while others suggest that he may have been mentally unstable. However, Brown was raised in a Calvinist family that was strongly against slavery and, after a lifetime of failed business ventures, he believed that he had found his calling to fight for abolition while attending an anti-slavery convention in Cleveland, Ohio in 1837. Brown's Calvinist faith, which includes the notion of predestination—that all events in one's life are willed by God, or fated—may have also influenced his decision to become a radical abolitionist.
Though Brown's act of defiance is one of the most memorable in history, its effectiveness could be the results of both timing—the North and South were strongly divided over the westward expansion of slavery and the Fugitive Slave Act of 1850, which demanded that slaves who escaped to the North be returned to the South—and the fact of Brown's being a white man who had led a raid with other white men. Previous revolts against slavery were insurrections led by slaves. A white man's commitment to die to end an unjust system likely drew more attention and was probably taken more seriously.
The anti-slavery convention at which Brown was moved to radicalize is also important. Anti-slavery conventions, which were held frequently and internationally (London was the site of an anti-slavery convention which included the suffragists Lucretia Mott and Elizabeth Cady Stanton), drew large crowds and included the voices of men and women who had escaped from slavery—particularly Frederick Douglass, who began his career giving speeches about his life story at anti-slavery conventions, leading to the publication of his narrative.
Douglass worked closely with William Lloyd Garrison, the Boston-based publisher of The Liberator, an abolitionist newspaper. Douglass went on to publish his own abolitionist newspaper, The North Star. Abolitionist media was banned in some Southern states. Southern legislatures even attempted to stop the dissemination of anti-slavery newspapers and petitions through the mail. These attempts by Southern legislatures to prevent the dissemination of this literature—the most prominent of which was published by Douglass and Garrison—indicates that media was quite effective in changing some hearts and minds.
https://www.history.com/this-day-in-history/john-browns-raid-on-harpers-ferry