I like Bacall and all, but Hepburn was making movies long before Lauren ever started. She has a more extensive body of work, and though I like some of Bacall's films a lot, overall I'd have to say Katharine.
For those of you who like older movies with Bing Cosby-
The somewhat lighthearted Going My Way or the more serious sequel, The Bells of St. Mary's?
Edit- Crap, Sethite beat me to it. In that case, Mars. I love the Goddess of Love, but I need action in my life to ensure that things are done. Mars, for me, is the energy during the workday; whereas, Venus is the enjoyment and relaxation afterwards. I'd prefer to relax and enjoy life, but since it's the middle of the day with much work yet to be done, I'll have to go with Mars.
(Trying again) The somewhat lighthearted Going My Way or the more serious sequel, The Bells of St. Mary's?