I would like to think that it's Sara Peretsky. However, there so many others that could hold that title. She is my choice for now. And the runner up would be, in my opinion, Janet Evanovich. Following closely behind her would be Sue Grafton. I have read them all and others. Sara came out on top for me. I will always think that all the mentioned authors above and many unknown others are really exceptional writers in their own right.
Ruth Rendell (Barbara Vine) is pretty well known.
Ann Cleeves in the UK is rapidly making a name for herself as well.
How about Liz Evans also from the UK? She's promoted as the British Janet Evanovich, but she's better than that.
If you score quality over quantity, Ruth Rendell owns this discussion.
Reverse the equation, and it's Agatha Christie.
Sue Grafton and Val McDermid are just below them, I think.
I have not read Ann Cleeves (shame on me), but she's on my list.
Evanovich isn't my cup of whatever...a much better writer in the same vein is Sparkle Hayter.
Another underrated writer is Randye Lordon and her Sydney Sloane series, though Lordon may have hung it up...she hasn't had a new book in 5 years or so. If you like Laura Lippman, Lordon is well worth seeking out.