Okay so my picks from the 1940's:
1. That Hamilton Woman (1941)
The story of Lord Nelson and Lady Hamilton starring Vivien Leigh & Laurence Olivier. I love this movie. My only regret is that it's not in color.
2. Adam's Rib (1949)
Katharine Hepburn and Spencer Tracy is in fine form in this comedy. Just a hilarious movie that's a delight to watch.
3. Caesar and Cleopatra (1945)
Because I have this fascination with Queen Cleopatra of Egypt. Vivien Leigh's take on the role is very different from Elizabeth Taylor's. Also this movie is in brilliant color.
4. National Velvet (1944)
Elizabeth Taylor's break out role. It's a feel good story about a young girl, who can do anything she sets her mind to. A horse movie that also has a bit of a feministic message. In color.
5. Lassie Come Home (1943)
Because I just love that dog. Also in color.
6. Rebecca (1940)
Great Hitchcock movie with Laurence Olivier that keeps you guessing.
7. Little Women (1949)
This take on the classic novel stars Elizabeth Taylor and is in color.
8. Woman of the Year (1942)
Another hilarious Hepburn-Tracy comedy.
9. 21 Days (1940)
Another rare Vivien Leigh and Laurence Olivier collaboration.
10. Casablanca (1942)
I need to rewatch this movie. Might have been higher on my list if it hadn't been so long since I saw it. Ingrid Bergman & Humphrey Bogart.
Honorable mention to "The Philiadelphia Story" (1940) and "His Girl Friday" (1940) that are also excellent movies...
So that's my list.

Now what is yours?