Tom is not very honorable if we think of honor as having anything to do with nobility of purpose, concern for others, or a willingness to deal with things directly.
He bullies and evades, but he does not conquer or compromise.
Gatsby, at least, seeks to conquer his own deficiencies. He wants to be something better in the future than he was in the past.
Tom sets up Gatsby's murder - a cowardly act - while Gatsby, in death, receives his due and (passively) atones for his deceptions.
Perhaps one way to go about this is to discuss how Tom is dishonorable. There is little in Tom's character that shows him to be honorable. He treats people in the worst of manners, acknowledging them as means to ends as opposed to ends in of themselves. He abuses people physically and emotionally and never carries himself with any notion of honor. For Tom to be honorable would mean for him to believe in something larger than himself. Given his self- serving and self- indulgent manner, this is impossible to find. Tom mistreats everyone in his path simply because he is able to do so being the benefactor of extreme wealth and social power. In this light, Gatsby would have to be seen as more honorable. Gatsby is in love with a dream, a vision of reality that might not be immediately recognized, but something that animates him and provides a sense of reason in his being in the world. For Gatsby, this is what makes him so honorable. He treats Daisy, for example, in an almost reverential manner, unable to harm her or demand anything out of her even when it is so painfully evident that Tom is a brute and Gatsby is a good man. In this, Gatsby is honorable, something more and better than what the world of social climbing and manipulative betrayal deserves.
Jay Gatby is the most honorable because that is how Nick perceives him as. Gatsby is a wealthy and generous man who is striving to regain Daisy's love again and threw all those parties to get her attenion or the possibility of seeing her. Tom on the other hand, is just a arrogant, racist, and self-absored man who cheated on his wife.