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.
You should check out 'Nana'. It's not a typical romance manga, but it has deep and complex romantic relationships. The story is completed and very engaging.
One great romance manga is 'Fruits Basket'. It tells the story of Tohru Honda who lives with the Sohma family, some of whom are cursed to turn into animals of the Chinese zodiac when hugged by the opposite sex. It has a wonderful blend of romance, comedy, and drama. The relationships between the characters are complex and deeply emotional, making it a very engaging read.
I think 'Kare Kano' is one of the best. It has a charming story and well-developed characters. The plot progresses smoothly, and the romance is heartwarming.