Citizen Kane aside, what do you guys pick? American Beauty is a solid choice.

Off the top of my head, The Maltese Falcon.

Among recent films, The Return is an astonishing debut effort.

Howzbout RESERVOIR DOGS, EL MARIACHI, HARD EIGHT, BOUND

uhhh... ummm... jeez... i don't know who the BEST would be, but here's a few debuts that made a significant impression on me...

Lynne Ramsay for Ratcatcher
David Gordon Green for George Washington
Paul Thomas Anderson for Sydney (AKA Hard Eight)
Wes Anderson for Bottle Rocket

Night of the Living Dead

Richard Kelly - Donnie Darko

I'll definitely agree with that one. Here's a few others: Blood Simple (The Coen Brothers), Duel (Spielberg), Pi (Darren Aronofsky) and The Terminator (James Cameron).

Richard Kelly -Donnie Darko
Wes Anderson - Bottle Rocket
Zach Braff - Garden State
John Hughes - Sixteen Candles
Phil Joanou - Three O' Clock High
Brad Bird - Iron Giant
Christopher Nolan - Following

I thought James Cameron's director debut was that film about flying fish.

ARG! I can't believe I missed that one!

You mean Pirahna II: The Spawning? Sort of. Actually, he only worked on it for a week or so before producer Ovidio Assonitis stepped in and took over (I guess he though he could do a better job?).

Howabout best debut from a director who made nothing but crap afterwards?

Jan De Bont - SPEED

Virgin Suicides

tao of steve
you can count on me
say anything

A few others:

Night of the Hunter - Charles Laughton's only directorial work.
Sex, Lies, and Videotape - I wonder if any other debut effort has won the Palme d'Or?

And assuming shorts don't count, I doubt anyone could top either Truffaut's Les Quatre Cent Coups or Godard's À bout de souffle.

Michel Gondry - Eternal Sunshine of the Spotless Mind

A crappy movie, made after he made the even crappier Human Nature.

Some "vanilla" favorites:

John Wayne for one of my all-time favorites The Alamo (1960). Where's the long version on DVD?!! Grrr........

Kevin Costner for Dances With Wolves

Ridley Scott for The Duellists

and for a more recent gem:
Asif Kapadia for The Warrior (2001)

I stand corrected. About the debut part, anyway.

Fair enough. Now go out and rent Human Nature.

Darren Aronofsky for Pi
Zack Snyder for Dawn of the Dead 2004
Sam Raimi for The Evil Dead
Guy Ritchie for Lock, Stock and Two Smoking Barrels
The Hughes Brothers for Menace II Society
John Singleton for Boyz n the Hood

Man there are tons...

No love for Human Nature. I personally enjoyed it, although it's the silliest a lightest effort from writer Charlie Kaufman.

As for other great debuts:

Alex Cox - Repo Man
Spike Jonze - Being John Malkovich
Danny Boyle - Shallow Grave
Rob Reiner - This is Spinal Tap
Shane Carruth - Primer

I think John Huston and Francois Truffaut are the clear winners here. Especially since neither director topped their debut (in my opinion).

Laughton with Night of the Hunter is the clear "winner" to me, and that's even if you include Kane.

I'm amazed no one brought up Harold Ramis/Caddyshack