Bad list, bad poll.
As well as others above have mentioned, your list also lacks:
-Kevin Spacey
-Dustin Hoffman
-Tom Hanks
-Marlon Brando
-Al Pacino
-Peter Sellers
-Gene Hackman
-Jack Nicholson
-Anthony Hopkins
Nobody seems to have mentioned any female actors...I know there aren't many really good ones around the big time in hollywood at the moment but is the omission a result of sexism or perhaps simply ignorance? Maybe the poll title should read 'best male actor'. So here's a few worth adding:
-Elizabeth Taylor
-Katherine Hepburn
-Emma Thompson
-Faye Dunaway
-Diane Keaton
The men and women on my list are *proper* actors, the poll's list includes some real talent but with the exceptions of Jackson and De Niro nobody close to being worthy of the title 'best actor'. Seriously, I love Stargate SG1 but Richard Dean Anderson!?!?!??
Edit: noticed that poll has been amended and withdraw some of my initial harshness.