The phrase "most slack-spined" means someone who lacks determination or backbone. The antonyms of this phrase could be words like determined, persistent, firm, resolute, unwavering, and strong-willed. These words express the opposite of being weak or indecisive, which is what "slack-spined" implies. Determined and persistent people are driven and committed, while firm and strong-willed people are reliable and sure of themselves. By using these antonyms, we could describe someone who is a true leader, with a strong sense of purpose and unwavering conviction in their beliefs. In contrast, "most slack-spined" is not a term that would be used to describe a successful and accomplished individual.