All Time -
Male - Al Pacino
Female - Meryl Streep
Currently -
Male - Denzel Washington
Female - Helen Mirren
Honourable mention......
Male - Kevin Spacey, Don Cheadle, Tim Roth, Chiwetel Ejiofor, Edward Norton, Terrence Howard
Female - Toni Collette, Charlize Theron, Alfre Woodard, Susan Sarandon, Scarlett Johansson