Do you mean best ever or just working today?
As for best actors ever i would definitely include the following:
Jack Lemmon - he was so versatile he could play anything!
James Stewart - same goes for him!
George C. Scott - a terrible overlooked and often forgotten actor, who always proved to be one of the greatest ever!
Marlon Brando - hell, he is Marlon Brando, how could someone make a list of the greatest ever and not include him!
Werner Herzog - probably one of the greates german speaking actors ever!
Robert Duvall - okay, i know he actually still works today, but he definitely belongs to this list as well and he is so often forgotten to mention!
Jack Nicholson - a legendary man, both as an actor and in real life, to anyone who thinks he always plays the same, watch: "One Flew Over the Cuckoo's Nest", "Chinatown", "The Shining", "Batman" and "The Pledge" and then still tell me he always plays the same, though i probably wouldn't believe you
Some others, who i just want to mention quickly, but nonetheless deserve a place in this list:
Robert De Niro, Al Pacino, Dustin Hoffman, Peter Sellers....and some others i forgot right now
As for still working:
Bill Murray - for me he is in the same category as Jack Lemmon or James Stewart, he can do anything and is always brilliant, he also belongs in the category of greatest actors of all time!
Johnny Depp - forget his rather average performance in "Pirates of the....", watch "Dead Man", "Fear and Loathing in Las Vegas" or "Ed Wood" instead!
Russell Crowe - maybe a jackass in real life, but an extremely versatile and talented actor, don't know if he will be on a list of greatest of all time, but definitely one of the best currently working actors, just watch "Romper Stomper", "L.A. Confidential", "The Insider" or "Gladiator", hell he even was good in "Proof of Life"!
Joaquin Phoenix - just watch any movie with him you want and he always delivers a great performance, okay the movies aren't always great, but he is!
Woody Harrelson - such a brilliant actor and so extremely overlooked and forgotten from the public, it's a shame, he is an amazingly talented guy, just watch his performances in "White Men Can't Jump", "Natural Born Killers", "Larry Flint" or "The Thin Red Line"!
John Turturro - an amazingly versatile actor, in fact he is so good, that you often don't even realize it's him, you only see the character he portrays!
John Goodman - heavily
depends on the movies you watch, but he can be an extremely powerful and intense actor!
Some other, who i just want to mention quickly, which doesn't mean that they are less talented tough, but i'm just terrible tired right now and i need sleep
Steve Buscemi, Jeff Bridges, Elias Koteas, Edward Norton, Robert Downy Jr., Jim Carrey, Kevin Spacey, Denzel Washington, Sean Penn, Nick Nolte, Bruce Willis and Harvey Keitel