I've been fascinated with spanking ever since I started reading historical romance books as a teenager. I read the spanking scene in Defy Not the Heart by Johanna Lindsey so many times, the book practically fell open to that page!
My favorite spanking scene from a mainstream romance, however, is, without a doubt, from Gypsy Lord by Kat Martin. In the story, the heroine has embarrassed the hero in front of his clan, and knows that the only way he will save face with his people is if he punishes her, so she convinces him that he must give her a spanking. Which he does, in front of everyone! Talk about hot! Whew!!!
Now, whenever I pick up a mainstream romance, I always hope that the handsome hero will put the feisty heroine over his knee for a good, sound spanking! In fact, I'm currently reading a book where the heroine definitely deserves one!
If you want a list of mainstream novels, romance and otherwise, with spanking in them, this Spanking Bibliography put out by CF Publications is the most comprehensive I've found!