Since this category has already been discussed at length, Id like to steer away from writing a review and rather focus on the actor/star category.
I agree with milinddesai and roshanferraris choices. But most of these choices are from the present day generation of actors, we might be missing some really good actors from the previous generations. The lists being mentioned are mostly from the actors who have starred from the 70s.
Great actors
Al Pacino
Robert De Niro
Dustin Hoffman
Jack Nicholson
The fifth one is a bit difficult choice between Kevin Spacey, Edward Norton, Sean Penn, havent seen a lot of Sidney Sheldon but hes good too.
Denzel Washington, thought he was known for his acting skills for quite long really changed his luck when Remember the Titans changed his image from being just an actor to a Box office winner. Most of his movies have done really well after this movie.
Five stars, these guys have been around for quite long except Clooney who was popular with his TV series ER when he entered films.
Tom Cruise
Mel Gibson
George Clooney
Brad Pitt
Will Smith
and Johnny Depp
And the Actor/Stars who balance the act of doing box office friendly movies along with performance oriented roles. Denzel would be be apt for this category but off late his movies are more suited for box office appeal.
Morgan Freeman
Sameul L.Jackson