Best five directors and their best work so far ...

[Vincent Farrell]

Steven Spielberg - Schindlers List
Michael Mann - Heat
John Woo - Face Off
Quentin Tarantino - Reservoir Dogs
David Fincher - Se7en

[John McFetridge]

John Sayles and any of his movies really, but let's say Lonestar, City of Hope and Matewan.

[Dana King]

These guys are so good I can't pick just one movie but here's my list, in alphabetical order:

The Coen Brothers - Fargo, Miller's Crossing, Blood Simple, and, of course, The Big Lebowski
Clint Eastwood - Unforgiven, Mystic River, Million Dollar baby
Martin Scorsese - GoodFellas, Casino. (I would say The Departer, but John McFetridge is following this thread, and I know better.)
Ridley Scott - Alien, Blackhawk Down, Thelma and Louise
Steven Spielberg - Saving Private Ryan, Schindler's List, Jaws.

[Elizabeth Zelvin]

Ang Lee, John Sayles, Mira Nair, Zhang Yimou, Peter Jackson

[Adrian Hyland]

I'm with Elizabeth on this - Zhang Yi Mou - I'd add Chen Kai Ge - their early work, in both cases.

Adrian

[Dwight]

John Woo-The Killer. Completely changed the way action movies were made.
Tarantino-Pulp Fiction; again, changed the industry (for better or worse.)
Scorsese-Departed or Good Fellas, a toss up.
Coen Brothers-Fargo, and No Country for Old Men. Sheer Brilliance.
Ernest Dickerson-Juice. Ok, i'll admit, this last one is a personal favorite of mine, but only because of the character of Bishop (played by Tupac Shakur.) The single most sociopathic hoodlum you will see in a movie ever. If you haven't seen it, dont say anything until you've watched the movie.