noun, noun which may take the genitive case particle 'no'
the best in the land; unrivaled in the country
Emphatic, somewhat literary expression meaning 'the very best in the whole country'. Often used in historical or dramatic contexts, or in set phrases like 天下第一の～.
He was praised as the greatest swordsman in the land.
This castle boasts the strongest defenses in the whole country.
天下一 is a more common and slightly less formal synonym, also meaning 'the best in the world/land'. 天下第一 is more emphatic and literary.
日本一 means 'the best in Japan' and is more modern and neutral. 天下第一 carries a classical, dramatic nuance.
Compound of 天下 (tenka, 'the whole country; the world') and 第一 (daiichi, 'number one; the best'). The exact historical derivation is uncertain, but the phrase follows classical Chinese-influenced word formation.