Tom's first master is Arthur Shelby.  He is comparatively benevolent in his treatment of his slaves, is known as a "kind master", and is reluctant to sell Tom when he finds himself in financial straits.  Ultimately however, his business sense takes precendence over his "humanity", and he sells Tom to a slave trader in order to pay his debts.

Augustine St. Claire, Tom's second master, is sensitive and kind, somewhat of an idealist who opposes the idea of slavery and plans to eventually emancipate Tom.  He does not act quickly enough, however, and is killed before he can give Tom his freedom.

Simon Legree is Tom's final master, a brutal, depraved man who represents slavery at its worst.  Infuriated at Tom's steadfast refusal to be corrupted, Legree finally beats him to death.

In a moral hierarchy, St. Claire would be at the top, because he has thought about the reality of slavery and understands it is wrong.  Shelby, who is sensitive to a degree towards his slaves as people, would come second, while Legree, in his unspeakable cruelty, would be last.  All three men are irrevocably mired in the destructive institution, even Shelby, whose avariciousness takes precedence over his tendency to be kind, and St. Claire, who opposes slavery in theory but cannot bring himself to take action to extricate himself from the system and set his slaves free.  The men do differ, however, in their levels of morality and their respective senses of humanity.