I'd say 'Naruto' is a top choice. It has a rich world-building, diverse characters, and a conclusion that ties up most loose ends. Plus, it's had a huge influence on the manga industry.
One of the well - known ones is 'A Dream of Red Mansions'. It's a classic that delves deep into the complex relationships in a large family. The love stories within, like that of Jia Baoyu and Lin Daiyu, are full of melancholy and beauty. Another is 'Romance of the Western Chamber', which tells a vivid and passionate love story between Zhang Sheng and Cui Yingying against the backdrop of feudal society. There's also 'The Plum in the Golden Vase', which, while also exploring the darker aspects of society, has elements of complex relationships and romances.
Well, 'Nana' is often touted as a top romance manga. It delves deep into the emotions and relationships of the characters. The art style is also captivating.