Well it really depends on the era or time frame.
During the reign of Nicholas I, you had his daughters, Grand Duchess Alexandra Nikolaevna and Olga, Queen of Württemberg then their was his daughter in law Grand Duchess Alexandra Iosifovna.
During the reign of Alexander II, there was Grand Duchess Olga, wife of Grand Duke Michael Nikolaevich. Grand Duchess Maria Feodorovna, wife of Alexander III, then we have Grand Duchess Elizabeth wife of Grand Duke Serge. Grand Duchess Maria, wife of Grand Duke Vladimir however in her youth was a handsome woman, however I don't believe she was ever considered a beauty. Not positive.
During the reign of Alexander III, there was Grand Duchess Anastasia, daughter of Grand Duke Michael and Grand Duchess Xenia.
During the last reign of Nicholas II, there would be a few more beauties. Grand Duchess Tatiana Nikolaevna was considered the most beautiful of the tsar's daughters, although I always thought Grand Duchess Maria was prettier, Princess Irina really did fit the standards of beauty for her time, Natalia Brassova, Countess Brasova wife of Grand Duke Michael was considered very lovely by many.