The two characters that display the most ambition in this play are Macbeth himself and Lady Macbeth. They reveal the depth of their ambition in their willingness to murder Duncan in order for Macbeth to gain the throne, as predicted by the Weird Sisters.
They each confess their ambition to the audience -- Lady Macbeth in Act I, scene v, and Macbeth in Act I, scene vii.
Lady Macbeth receives a letter from Macbeth, telling her of the witches' predictions. Immediately, her ambition is inflamed. She says:
Glamis thou art and Cawdor; and shalt be
What thou art promised. . .
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements.
By determining that Macbeth "shalt be" King (what he was promised by the witches) and calling Duncan's entrance into Macbeth's castle "fatal," Lady Macbeth shows how readily she embraces any action necessary in order to make her husband King.
Two scenes later, Macbeth reveals that he is motivated by the same ambition as Lady Macbeth. In his first soliloquy of the play, he reveals to the audience that, though there are numerous reasons that killing Duncan is a really bad idea, he will go forward with the plan anyway. His reason? Ambition. He says:
. . .I have no spur
To prick the sides of my intent, but only
Vaulting ambition. . .
And so, both Lady Macbeth and Macbeth, by the end of Act I, have committed to a treasonous course of action: the killing of King Duncan in the name of ambition.