The sentence requires the superlative form of the adjective "beautiful" because it refers to "one of the" cities, implying a comparison among many. The superlative form of "beautiful" is "most beautiful".
The options provided are:
- more beautiful (comparative)
- beautifuler (incorrect comparative)
- most beautiful (superlative)
- beautifulest (incorrect superlative)
Therefore, the correct answer is "most beautiful".