Definitely 'Pride and Prejudice'. It's a masterpiece. The relationship between Elizabeth and Darcy is so well - developed. You can really feel the social constraints of the time and how their love overcomes them. Another is 'North and South' by Elizabeth Gaskell. It's about the clash between the industrial north and the genteel south in England and the love that blossoms between Margaret Hale and Mr. Thornton. It gives a great insight into the historical and social changes of that era.
I think 'Romeo and Juliet' by Shakespeare should be on the list. Although it's a tragedy, it's also a great love story set in historical Verona. Their love defies the feud between their families. 'The Time Traveler's Wife' by Audrey Niffenegger is also very good. It has a unique take on time - travel and love. The main characters' relationship spans different times in history, and it shows how love can endure through all kinds of challenges. 'The Duchess' by Amanda Foreman is a great historical romance novel that tells the story of Georgiana Cavendish, Duchess of Devonshire, and the complex relationships in her life.