Difficult question, as there're SO many, past & present, & dependent on 'categories'!
In no particular order or category......
Dame Maggie Smith
Dame Eileen Atkins
Dame Judi Dench
It used to be Kate Mulgrew because she played my favourite TV character, Captain Kathryn Janeway from Voyager, but since that is the only thing I've really ever bothered watch her in, I've switched allegiance to Gillian Anderson, who I not only love in The X-Files, but in BBC dramas too.
This is a tough one for me, both because there are so many strong female actresses on the small screen but also because female characters never seem to have their fair share of lead roles. Evangeline Lilly is certainly one of my faves, she was amazing in Lost. I've also been digging Kristin Lehman who stars in the Canadian crime show Motive.
Two of my favourites are Sigourney Weaver and Jennifer Lopez. Both have portrayed tough characters but other roles as well. Sigourney was fantastic in the Alien movies while Jennifer was powerful in the thriller The Cell.
Cate Blanchett because she is a brilliant actress who can be completely believable in any role and she is beautiful, intelligent, genuine and has the elegance and poise of the stars of old. Cate is also a fantastic ambassador for our country.
I have watched her for years, and knew her 22 years ago, when she was studying at Nida in Sydney.
She was a lovely human being then, as she is now.
So natural, and I have many fond memories of her.
Julia Roberts is so good; comedic, yet versatile and can have a quiet calm. Many good films, including 'Larry Crowne', 'Eat, Pray, Love', which was severely underrated, 'Pretty Woman', 'Steel Magnolias' and lately, with Ewan McGregor AND Meryl Streep in 'August: Osage County'. Kate Winslet is a fine actress, and was good in 'Holiday' and more recently, in 'Carnage' . Kristen Scott Thomas is very good, and just appears everywhere, in just about everything. The indomitable Michelle Pfeiffer, for all of her roles but lately for 'The Family'. Emma Thompson forever, for 'Love Actually', amongst others. Kyra Sedgwick; Diane Keaton, let's not forget 'Annie Hall'; Maggie Smith 'Prime of Miss Jean Brodie'; Judi Dench; Celia Imrie; Julie Walters 'Educating Rita', of course. Felicity Kendal; Audrey Tautou; Gena Rowlands and Susan Sarandon! Fin
Bette Davis was always my favourite My husband took me to see her on the stage in Sydney, for birthday present, when she was out here doing her Q&A show. I was to shy to stand up and ask her a question!
Now I also really like Dame Judi Dench, & Helen Mirren, & Cate Blancett.