зеботарин

Шаҳри Рио-де-Жанейро яке аз шаҳрҳои зеботарини дунё аст.
The city of Rio de Janeiro is one of the most beautiful cities in the world.
Шаршараи Игуасу яке аз зеботарин шаршароҳои рӯйи Замин аст.
The Iguazu Falls is one of the most beautiful waterfalls on the face of the earth.

Bonus:
These sentences are very similar, but notice that you can put the adjective either before or after the noun.

зебо: beautiful, pretty
зеботар: more beautiful, prettier
зеботарин: most beautiful, prettiest

Other words with the suffix тарин –
чуқуртарин: deepest
қадимтарин: oldest, most ancient
муҳимтарин: most important
бузургтарин: greatest

For more help with comparatives and superlatives see this worksheet: Comparatives and Superlatives