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 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.
'Nisekoi' is full of plot twists. Raku Ichijo is forced into a fake relationship with Chitoge Kirisaki to keep the peace between their rival gangs. But as the story progresses, there are so many twists and turns in their relationship. There are also other girls who may or may not be the girl with the key to Raku's locket, which was promised to his first love. It keeps you guessing all the time.
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.