I don't have one that I need to buy right away because my favorites are less known or dead. I tend to already have all the ones from the dead authors. I prefer hardcore horror, gory, nasty stuff. So my favorites would be Richard Laymon, Jack Ketchum, Edward Lee and some Brian Keene.
I don't have a #1 all-time favorite but I have several that I will read just about everything they write if that counts so here's my list.
James Patterson's Alex Cross, Women's Murder, Club, and Michael Bennett series
Alex Kava- maggie O'dell Series
John Sanford- Lucas Davenport Series
Johnathan Kellerman- Alex Deleware series
Catherine Coulter- FBI series with Savich and Sherlock
Janet Evanovich- Stephanie Plum series
Stuart Woods- Stone Barrington and Holly Barker series
Patricia Cornwell- Kay Scarpetta series
Like I said I read entire series from these authors in the series.