best ever; unparalleled in history
A somewhat literary or emphatic expression meaning that something is the greatest of all time, without equal in the past or present. Often used in praise of achievements, works of art, or exceptional people.
His performance is said to be an unparalleled masterpiece of all time.
This work is an unrivaled masterpiece throughout history.
Both mean 'unprecedented,' but 空前絶後 emphasizes that something has never happened before and will never happen again, while 古今無類 focuses on being the best ever without comparison.
無類 alone means 'matchless' or 'peerless,' but 古今無類 adds the temporal scope of 'past and present' for stronger emphasis.
A yojijukugo (four-character compound) combining 古今 (past and present) and 無類 (without equal). The exact origin is uncertain, but it follows a common pattern of classical Chinese-derived expressions used in Japanese for emphatic praise.