The Great Gatsby is sophisticated, mature fiction that reveals issues and ideas that concern human existence. As such, it doesn't really deal with simplistic, easily labeled, good and bad guys. People in the novel are, for the most part, mixtures of good and evil like actual people are.
Gatsby, for instance, loves like every human being would like to be loved. Gatsby is probably the happiest, and certainly the most goal-oriented person in the novel. He is relentless in his loyalty and dedication to Daisy. Yet, he's also a fool. He's naive, and dedicates his life to recapturing a relationship that never really was. His business practices are also questionable. We don't get many details, but he is partners with the man who fixed the World Series.
The second major player in the novel, Daisy, is multi-faceted, too. She appears at different times to be cynical, sarcastic, naive, greedy, innocent, dangerous, and harmless. She marries Tom for money, but she is a female in a patriarchal society--what other choices does she have?
None of these people are perfect, but they're not villains, either.
If there is a villain, it's Tom. One might have trouble coming up with redeemable characteristics that Tom possesses. He's ignorant, closed-minded, abusive, dominating, and hypocritical. Yet, even Tom has a tender side. When he mentions some good times he and Daisy had earlier in their marriage, he seems genuine, as well as honestly hurt that Daisy might leave.
I would say that the true villain would be the social order that is so highly parasitic upon one another in order to sustain their happiness. It seems that Fitzgerald is identifying the upper class and the social elite as the individuals that are primarily responsible for the poor and boorish behavior that causes them to engage in gossip, using individuals as means to ends as opposed to ends of themselves, and ensuring that the only value that is present is the one where individuals find the best party. In this world, the villain is this social order which causes individuals to behave in such a manner. Tom, Daisy, and Jordan are all fitting examples of such a setting.
In my opinion, the only real villain is Tom Buchanan. There are lots of other people in the story who do bad things, but to me, Tom is the worst.
Tom is the only one, in my opinion, who does not have an excuse for anything he does. He is not forced to have an affair with Myrtle. He is not forced to neglect Daisy so that she does not love him. He is not forced to make Wilson think Gatsby killed Myrtle. I can't really think of anything that makes him a decent character.
I do not think Gatsby is a hero. I think it's admirable that he chases what he wants, but what he wants is kind of pathetic (Daisy's not all that great) and the way he chases wealth is somewhat dishonest.
I like Nick better than anyone else in the book, but the only thing he does that is at all heroic is trying to get people to come to Gatsby's funeral.