her best movie??

Shadow of a Doubt, as you mention, is great.

I also highly recommend:

The Little Foxes (my pick for her best film)
The Best Years of Our Lives
Pride of the Yankees
Pursued
Track of the Cat

I was underwhelmed by The Men but some do like it, and it’s certainly interesting to see her acting alongside Brando. This pairing feels like a crossover between two generations of actors.

I have only seen her in three movies, yet I consider her one of my favorites of the silver screen.
The Pride of The Yankees
The Best Years of Our Lives
Shadow of a Doubt
I do have Mrs. Miniver and Little Foxes on DVR, so I will eventually get to them.

Shadow of a Doubt is excellent. So is The Best Years of Our Lives.

Hard to pick between the two. Both are great motion pictures.