My favorite romances are set either in Napoleonic-era Europe and Britain or in the Victorian/early 20th century period; my bias is reflected in my favorites.
Laura Kinsale. There is no other like her, and no one else could get away with some of her plots. Read them all, especially if you like stories with a lot of angst. My personal favorites are The Shadow and the Star and her recent novel Shadowheart.
Carla Kelly. Queen of the ordinary hero, she writes Regencies almost exclusively. I love, love, love her characters because they are sensible, not necessarily pretty, and have real flaws rather than writer-inflicted tics. My favorites include Summer Campaign and The Wedding Journey, but you can't really go wrong with any of hers.
Judy Cuevas/Judith Ivory. One of the finest prose stylists in Romance. Her settings are usually late 19th-early 20th century. Complex characters with flaws that sometimes stretch their likeability and original, hot sex scenes, rarer than you might expect in the genre. My favorite (out of print) Cuevas is Dance; my favorite Ivory is Untie My Heart.
Eva Ibbotson is in a class by herself, and is not usually marketed as Romance, but her adult novels like A Company of Swans are truly worth seeking out.
Georgette Heyer is, historically, of inestimable importance to the present Romance genre.
Other historical romance writers I like are Connie Brockway, Liz Carlyle, Jo Beverley, Gaelen Foley, Loretta Chase, Mary Balogh, Mary Jo Putney, Tracy Grant. Maybe later I'll try and compile a list of recommendations of their books, since some of those writers have massive backlists of varying quality.
Jennifer Crusie. Generally humorous books, recently categorized as "women's fiction" in some stores. Anything by her is good. One of my favorites by her is an old one called Anyone But You. Recently, I've loved Faking It the best.
Suzanne Brockmann. Romance/suspense. She wrote some standalone series romances but hit it big with a long series about Navy SEALs. More recently, she's spun off from that and seems to be increasing the suspense and decreasing the romance in her books, which I think is working well.
Mary Stewart's "contemporary" ranges over several decades. Her books are sometimes outdated in their depiction of gender roles, but I love them anyway. Favorites are Touch Not the Cat, The Moon-Spinners, This Rough Magic, and The Gabriel Hounds.
Sometimes, I like Susan Elizabeth Phillips. And sometimes I don't. But she's a big name in contemporaries.