Without a doubt, I'd have to say J.C. Wiatt (AKA Diane Keaton in Baby Boom). This was one of my absolute favorite movies growing up. I was five when it came out (do the math!) and absolutely obsessed. And I still am. Anyway, we all know the story. The ultimate yuppie career-driven city girl unexpectedly becomes the guardian to a baby girl. She has to learn to be a mother and juggle work with family. She experiences hurdles along the way (getting fired!, moving to the country!), and eventually realizes (epiphany!) that motherhood is the best job of all. Love love love this movie! Love Diane Keaton. And that baby is insanely adorable.
Honorable Mentions: A few more of our favorites...
Kate (AKA Tina Fey) and Angie (AKA Amy Poehler) in Baby Mama.
Donna (AKA Merly Streep) in Mamma Mia!
Mrs. Gump (AKA Sally Field) in Forest Gump.
Edwina (AKA Holly Hunter) in Raising Arizona
M'Lynn Eatenton (AKA Sally Field - again) in Steel Magnolias.