"Beauties" are pretty well listed above, but I'll still vote Una O'Connor as my favorite scream queen. Even if she's just yelling out from the witness stand about her hearing aid, I love seeing her get into a scene where she can raise her voice.
I've been collecting Una's Evil Twin Sister, Tiny Jones, when I finally discovered her name (as the Widow Tillane's maid, busting Victor McLaglin's chops over wiping his muddy boots). Both of these women's roles (and poor Tiny - was she the Queen Of Uncredited Screaming & Yelling? I think so) had them constantly pitted against heavyweights in so many films, yet they had this "It's the size of the fight in the dog" attitude.
I wish I'd grown up with a greater appreciation for those two as well as all of the frequently overlooked suppporting actors.