Say Macbeth is like so.... where as Banquo is like so... and Macduff like so... (this is contrasting)
When comparing you are going to have to look at how each reacts in certain situations compared to Macbeth. For instance, Banquo reacts to the prophesies of the "weird sisters" with suspicion, mistrust and a certain level of foreboding whereas Macbeth reacts with elated disbelief, hangs on every word and sees it as the beginning of great things. And from his reaction the things said by the sisters were ambitions that were within him already whereas Banquo had no real ambitions to become father to a line of kings before that. That's why he says "Speak to me who neither begs nor fears your favors nor your hate" - or something like that.
And use quotes. A good essay always has good quotes woven into it. But don't go wild with them. Just place one or two here and there to support a point.
Also, personally I have never viewed Macduff and Banquo as heroes. They lack the heroic potency of a Shakespearean hero.
Shakespearean heroes are dense characters and these two are pretty straightforward. With Shakespeare there's always more to a hero than meets the eye. So I would say, it is clear beyond doubt that Macbeth is the hero of the play. But the key is - he's a tragic hero, one who ends up a fallen hero.
And aren't such heroes always the best?
Banquo and Macduff would have been boring heroes, clean, cut and straight as they are. They would have been so predictable. Whereas with Macbeth you could never know what he was going to do.
In the beginning we are introduced to this good, noble guy who's fatal flaw is ambition and we hope he won't go through with Duncan's murder but he does; we pity him, hope for his redemption even...