So many greats.. from this solid decade it would have to be Treasure of the Sierra Madre (1948)

Although Red River ('48), They Were Expendable('45), Objective, Burma! ('45), White Heat('49),are close.

I'm going with the two obvious answers...not because I they're the only films from the 1940's that I've seen, but because they honestly are my favorites for the decade, and up there with my favorites of all time: Citizen Kane and Casablanca.

-JP

The Third Man, followed by The Best Years of Our Lives.

I also have a real soft spot for Sergeant York.

Gaslight (both versions) are near the top of my list

They Died with Their Boots On (1941)

So many choices, but my favorite turns out to Ernst Lubitsch's To Be or Not to Be (1942)

Okay, let's get these out of the way first:
Buck Privates
Hold that Ghost
Who Done It?
The Time of Their Lives
Buck Privates Come Home
The Wistful Widow of Wagon Gap
Abbott & Costello Meet Frankenstein

White Heat
It's a Wonderful Life
Miracle on 34th Street
This Gun for Hire
The Maltese Falcon

There are so many classics that I haven't seen.

Citizen Kane, It's a Wonderful Life, and Rebecca.

The Wolf Man
Abbott and Costello Meet Frankenstein
Treasure of the Sierra Madre
The Maltese Falcon

Casablanca

The perfect film noir from the decade of film noir. I'll also give an honorable mention to The Big Sleep and Notorious since nobody has mentioned them yet.

The Lost Weekend

There's so many good movies coming out from the decade. But if I had to pick, probably Citizen Kane and Casablanca. I haven't watch The Maltese Falcon yet.

Have to go with Casablanca...the scene where Rick lets the poor girl win at roulette gets me every time

There are so many good films in that decade making it hard to chose. Right now I'm on a Golden Age of Mexican Cinema (Epoca de Oro) kick and have recently viewed Enamorada (1946) and La Perla (1947)so I'll name these. Both were directed by Emilio Fernandez and photographed by Gabriel Figueroa and starred Pedro Amendariz. Both are exquisite and well worth your time if you can gain access to them. I don't believe there is a legitimate R1 release of either which is a shame.

These two along, with Maria Candelaria 1944, also directed by Fernandez were shown and honored at Cannes in 2005 as part of their Cannes Classics series. Figueroa won the Cannes cinematography award in 1946. He is Mexico's most famous cinematographer and studied under Gregg Toland and is the bees knees.

From the ones mentioned, The Treasure of the Sierra Madre, The Third Man, Casablanca, The Maltese Falcon, White Heat and Red River are all amongst my favorite movies, but the first thing that came to mind for me when I saw the topic was:

The Philadelphia Story (1940)

The Grapes of Wrath. Also, High Sierra, Double Indemnity, Out of the Past and the first four Road movies.

Double Indemnity and Out of the Past, definitely. Also, Maltese Falcon (I'm a noir fan) Casablanca, Philadelphia Story, and Notorious.

It's amazing how many of my all-time favorites came out in the 40's.

Now Voyager

Arsenic and Old Lace is probably my all-time favorite movie.

Some others (some mentioned) off the top of my head that I love:
The Third Man
The Philadelphia Story
Suspicion
Notorious
Mr. Blandings Builds His Dream House
His Girl Friday
The Lady Eve
Shadow of a Doubt
Rope
The Big Clock

And if I sat down and thought about it, I could probably come up with 100 more.

(Of those, Out of the Past is my personal favorite film of the '40s.)

Either a lot of people are afraid to list Casablanca and Citizen Kane, or they wouldn't even put them in a top-10 list for the entire decade. Either way, I'm confused.

Also, add Rope to my list. Not sure how I forgot that'n. One of my favorite Hitchcock movies.

-JP