Flat: Tom Buchanan. Tom Buchanan is a one-note buzz kill. Wherever he goes, misery follows, be it at his mansion the first night Nick arrives for dinner or at Myrtle's apartment. He even manages to deflate the festive air around him at Gatsby's party. A narrow-minded, brutal, upper-class racist, he acts predictably, and as Nick points out, he has a limited intelligence.
Round: Gatsby. Through Nick's eyes, Fitzgerald portrays Gatsby as a flawed and complex character, a self-inventing person of grandeur. He is both a criminal and a generous dreamer; a man from nowhere who dares to aspire to have it all.
Static: Daisy. We first meet Daisy lying on a sofa on a hot summer day, oppressed by life and reacting to others rather than forging her own destiny. She's weak throughout the novel and unable to change or break away from Tom.
Stock: Mr. Wilson. He's the stock colorless cuckolded husband. Beyond owning a gas station and being married to Myrtle, we learn very little about him.
Confidante: both Nick and Jordan. Gatsby confides in both these characters as he pursues his dream of reuniting with Daisy. Gatsby approaches Nick by first telling Jordan his story. Gatsby will later confide directly in Nick. Both function as the observers and supporters of the main drama unfolding, inviting confidences through their quiet ways of standing just outside of the limelight.
I think the flat role would really fit Tom pretty well. He is basically in there to be a boorish, nasty, racist bastard with tons of money. You don't get a more thorough look at him.
As a confidante, Nick certainly fits the role, he is telling the story, treating the reader as a confidante.
I think you could make the argument that Daisy is static. She maintains the constant role as a manipulator, no matter who she is around, no matter who she is manipulating, she never relents and never changes her tune. She's used her beauty her whole life and she doesn't stray from that path.