Frailty

Mike Nichols (for Who's Afraid of Virginia Woolf?) is a worthy candidate. Like Welles, he was a show business veteran who had directed extensively in theater before helming his first movie. Still going strong at age 73.

uh, Un Chien Andalou (1929) anyone?

and I'd say Maya Deren's Meshes of an Afternoon (1943) is equally impressive for a debut.

Also Errol Morris' Gates of Heaven (1978)

and mucho agreement on Laughton's astounding Night of the Hunter

Off the top of my head of those not mentioned..

Terrence Malick - Badlands
John Cassavetes - Shadows
Nicholas Ray - They Live by Night
George A. Romero - Night of the Living Dead

EL MARIACHI is sick!

Love Caddyshack too

As for the mini Gondri/Kaufman discussion... "Human Nature" may not be a great film, and certainly not the greatest film debut, but I chuckle out loud just thinking of it! A very funny movie, absolutely worth seeing, and much less self-consciously aren't-I-clever-Ma? than Kaufman's other scripts (though you won't mistake it for anyone else's).

I'm not even going to venture a candidate for best ever directorial debut, but one that I saw recently on DVD very much impressed me: THE RETURN (Vozvrashcheniye), by Andrei Zvyagintsev. http://imdb.com/title/tt0376968 It's on DVD in Region 1, and well worth seeing.

It should have come to me sooner, but I was reminded of this one after checking out TCM's schedule for the month of June:

12 Angry Men by Sidney Lumet

Everyone overlooking EL MARIACHI ?

no, i don't think they were....

see post 3 and 30

STRONGLY AGREE with Night of the Hunter

Roman Polanski - Knife in the Water

A truly awesome film.

I'd also nominate
Targets by Peter Bogdonavich
Shadows by John Cassavetes
Who's Afraid of Virginia Woolf? by Mike Nichols
Being John Malkovich by Spike Jonze
Easy Rider by Dennis Hopper

Zach Braff - Garden State

Blue Collar - Paul Schrader