Paul,
I'd have to say that left handed in the Ladies Elizabeth Horne would be #1, as she's beaten Susan, Elaine, Dawn, Allison and Jo - all of them a few times. She's never been beaten on the left in the UK, and has done well abroad - 3rd at the World's in 2001 (L/H), 3rd at the World Challenge 2009 (R/H), and 6th (both hands) at this years World's.