Wow, this is a great question that can produce a variety of answers depending on an individual's taste!
Malcom - For this role, we need an actor young enough to play the frightened son who flees but old enough to eventually reclaim the throne. I hate it when this role is cast with someone much too old. I would choose Asa Butterfield. He is a young-looking fifteen-year-old who has starred in The Boy in the Striped Pajamas and Hugo.
Donalbain - Donalbain has to be younger than Malcolm. This, too, is a minor role, so I would choose Chandler Canterbury. He is around 13 years old and worked with Nicolas Cage in Knowing and as a young Brad Pitt in Benjamin Button.
Banquo - Banquo is a contemporary of Macbeth, so he should be in his late 20's or early 30's. Joseph Gordon Levitt is 29 years old and is an accomplished actor. His boyish looks and air of kindness and honestly would make him a great choice for Banquo.
Hecate - Helena Bonham Carter is a superbly talented actress who has proven herself in both Shakespeare and in popular film genres. She did a wonderful job as Ophelia in Hamlet and as bizarre characters in the Harry Potter movies and in Alice in Wonderland. She would pull off a scary, creepy Hecate any day.
5) Fleance - Fleance is the youngest character listed here, and I have cast his father as Joseph Gordon Levitt. He must be old enough to accompany his father on a journey but young enough to be incredibly vulnerable. CJ Adams from The Odd Life of Timothy Green seems to be a perfect fit.
6) Lennox - Lennox is basically a supporting role, so Chris Cooper is my choice here. He has worked in successful supporting roles in a long list of movies that include Seabiscuit, Syriana, and the Bourne series. He also appeared in a Shakespeare adaptation of The Tempest.
7) Ross - In some film versions Ross is implied to be working with Macbeth. Henry Cavill is a British actor with a scruffy look that could play this role well. I loved him as Charles Brandon in The Tudors.
You can find photos and more information about these actors at the webside listed below.