Mr. Impossible by Loretta Chase is the very first book I've ever given a perfect review score to, and I still love it. The hero is simply wonderful, funny and strong despite his weaknesses, and the heroine is more than his match.
Only for a Knight by Sue-Ellen Welfonder is part of a series set in Scotland. Her setting serves almost as another character in the story, yet doesn't intrude on the romance, a fine line to tread. Then there are her sexy heroes, sometimes a little magic besides.... All in all, a pretty good example to study.
Sealed with a Kiss by Pamela Morsi is set in rural/small-town historical America, with a cast of unforgettable characters. Her details enhance the story, adding to a reader's enjoyment of a finely-told romance.
Devil's Heiress by Jo Beverley, one of the best historical authors around, this one features a heroine strong for the time period, yet not unbelievable, and a hero who won't let her down. What more can a reader ask for?