¿which is your favorite movie vs what is your favorite movie?
which one is your favorite movie can also be an option? :S

Question

Answer

"which one" is: out of A, B, C,... which one of these is your favorite?
"what is" is: out of every movie ever, what is your favorite?

Answer

Thank you Dana :D

Answer

In general "What is your favorite movie?" is correct.
"Which (one) is your favorite movie?" is correct if you give the person a list of movies to choose from.

For example:  "I have the movies Shrek, Home Alone, and iRobot. Which one is your favorite?"

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