The one author I would totally recommend would be Jeaniene Frost. She's the author of the Night Huntress series - Halfway to the Grave and One Foot in the Grave. Her third novel, At Grave's End will be coming out in December, but we're all hoping it'll be out sooner.
I don't know if I can pick. I love Charlaine Harris and MaryJanice Davidson. I also buy Savannah Russe and Laurell K. Hamilton as soon as they come out and budget allows. It's a hard call!
Ack! Only one? You are jesting, surely! :D I'm going to have to say Laurell K. Hamilton, Sherrilyn Kenyon, Gena Showalter, Jackie Kessler, Adrian Phoenix, Lynsay Sands, Lilith Saintcrow, Richelle Mead, Jennifer Rardin, PC & Kristin Cast, Jeaniene Frost, Lisa Shearin, Michelle Rowan, Marta Acosta, and I have a lot more if anyone wants to hear them. :)
Alot of my faves have been mentioned already, but two others that I enjoy are Kim Harrison (the Rachel Morgan series is terrific) and Jasmine Galenorn (I have only read the first book - Witchling - so far, but it seemed like an interesting storyline).