The most mathematical flag: Nepal
Do you like geometrical constructions? Then you should like theflag of Nepal, which is the only non-rectangular flag of a nation, is actually defined bya Euclidean ruler and compass constructionin their constitution. See astep by step drawing of the constructionorthis video from Numberphile.
The flag of the state of Ohiois also non-rectangular, but sadly their laws only describe it as “burgee-shaped”. It might be possible to extract a construction fromthe picture in this brochure from the Ohio secretary of state, but it looks like many of the points are specified but their position in a larger rectangle. Ohio’s flag is analytic geometry; Nepal’s is synthetic.