My favourite is Faro's Daughter. Deborah Grantham is a superb heroine, spirited, honourable and loyal. Max Ravenscar - the wealthy, saturnine, arrogant Mr Max Ravenscar - is a terrific hero and a worthy adversary. The battle of wits and words that ensues between these two, who loathe and despise each other's actions while discovering aspects of each other's character that surprise then attract, is a joy to read.
That would be reason enough to enjoy this book. But for me, what makes it stand out is the wonderful character of Deb's Aunt Lizzie, Lady Bellingham, who accepted responsibility for Deb and her brother Kit after their parents' death and was then forced to open a gaming house in order to make ends meet. Aunt Lizzie's panic and bewilderment at the events erupting around her is both touching and hilarious.
It's a wonderful book. If you haven't read it, I'd recommend you get a copy at once. Then there's Devil's Cub...