Who is the least honorable character?
I think that Steinbeck would reject any one person being seen as without honor. The condition of the Great Depression and economic hardship made so many people have to resort to savage means of living. This is devoid of honor because one feels compelled to survive by any means necessary. Steinbeck would feel that the conditions that dictate this as a means of living is, in itself, dishonorable.
With that in mind, I would suggest "the bank" is the most dishonorable character in The Grapes of Wrath. The bank is seen as a force without a countenance and is basically driven by only profit. In chapter 5, this is shown in the most brutal manner, a realm in which honor is impossible. The tractors that the bank hires to plow over and destroy the farmers' lands are seen as " snub-nosed monsters, raising the dust and sticking their snouts into it, straight down the country, across the country, through fences, through dooryards, in and out of gullies in straight lines." The bank commands these instruments of destruction to do their worse to land that has been in the family of farmers for generation. These farmers are forced to see the dishonor of capitalism that the bank embodies.
The bank is shown to be a force that lacks honor, as it fails to look the people that it is displacing in the eye: "Fellow was telling me the bank gets orders from the East. The orders were, 'Make the land show profit or we'll close you up." The bank is the most dishonorable character because it lacks a human identity. It is "the monster" that is driven through profit with nothing else as its concern:
"The bank — the monster has to have profits all the time. It can't wait. It'll die. No, taxes go on. When the monster stops growing, it dies. It can't stay one size....." "It's not us, it's the bank. The bank isn't like a man. Or an owner with 50,000 acres, he isn't like a man either. That's the monster...."
"The monster" that the bank embodies dishonor because it does not take anything else into account. Human misery and pain do not register as it strives to establish more profit. For Steinbeck, the condition that enables profit at all costs is a dishonorable one.