It's been over six years since I wrote this, and so many things have changed since then.
As of this post, I think Hepburn and Davis might tie if I were to rank them. Hepburn is stellar in screwball comedy, probably the best actress I've seen tackle such material in Classic Hollywood; BRINGING UP BABY (1938) will always be one of my favorite comedies. Her later work, like SUMMERTIME (1955) and the TV movie LOVE AMONG THE RUINS (1975), are equally good, giving me an appreciation for the consistency of Hepburn's work, even as she aged and some of her peers fell into obscurity.
Davis had the biggest hits and showed the most versatility during Hollywood's Golden Age. I've always had a soft spot for JEZEBEL (1938), and ALL ABOUT EVE (1950) is hands down one of my favorite movies ever. Davis dove into "grande dame guignol" in the 1960s and I'm glad she did, because I wouldn't liked to have missed BABY JANE (1962), CHARLOTTE (1964), or THE NANNY (1965). Davis employs a theatricality in her roles that I find intriguing, even if it hasn't aged very well and places her firmly in an acting style long out of fashion.
Crawford has always held a place in my heart. MILDRED PIERCE (1945) is a great film, to me at least, as are many of her Warner Brothers movies in the mid-to-late-40s and early-50s. I give Crawford credit for one thing: she changed with the times, finding movies with "audience identification" and giving her public what they wanted from her. She was, first a foremost, a star and she was devoted to that image and life. When on her A-game, Crawford could be a very good actress.
My opinion has changed the most on Stanwyck. After years of flat despising her, my view changed after watching DOUBLE INDEMNITY (1944) and SORRY, WRONG NUMBER (1948). While Stanwyck might've lacked the mainstream prestige of Hepburn, Davis, and Crawford, she possessed a versatility greater than any of them. She floated easily between genres and played a wide variety of characters.
See, time changes things.