Interesting list, didn't expect to see Godard on it. I couldn't separate it by country and region. I know we had a top ten actresses thread on the old, old site, but this will do for the noobs.

Off the top of my head, in no strategic order.

Marlon Brando (Last Tango in Paris)
Charlton Heston (Ben-Hur)
James Cagney (White Heat)
Humphrey Bogart (Treasure of the Sierra Madre)
Spencer Tracy (Father of the Bride/Adam's Rib)
Buster Keaton (Steamboat Bill Jr)
Katherine Hepburn (Suddenly Last Summer)
Natalie Wood (Spelndor in the Grass)
Jack Nicholson (Ironweed)
Gong Li (Ju Dou)
Julianne Moore (Far From Heaven)
Naomi Watts (King Kong/Mulholland Drive)
Clint Eastwood (Million Dollar Baby)
Gary Cooper (Meet John Doe)
Greta Garbo (Camille)
Miram Hopkins (Design for Living)
Dustin Hoffman (Midnight Cowboy)
Lee Marvin (Point Blank)
Clark Gable (San Francisco)
Nicole Kidman (Moulin Rouge)
Peter Lorre (M)
Vincent Price (His Kind of Woman/Masque of the Red Death)
Boris Karloff (Bride of Frankenstein)
Gene Kelly (Les Girls)
Barbara Stanwyck (The Lady Eve)
Ingrid Bergman (Voyage in Italy)

I'll add as it becomes necessary.

Also added my favorite performances from everyone selected, not necessarily the best film they were in.