blockbuster vs smash hit

blockbuster and smash hit both are nouns.

Word	Noun	Adjective	Verb	Adverb
blockbuster	Yes	No	No	No
smash hit	Yes	No	No	No

As nouns, blockbuster and smash hit are synonyms defined as:

blockbuster and smash hit: an unusually successful hit with widespread popularity and huge sales (especially a movie or play or recording or novel)

Other synonyms of blockbuster include megahit.

blockbuster (noun)	smash hit (noun)
an unusually successful hit with widespread popularity and huge sales (especially a movie or play or recording or novel)	an unusually successful hit with widespread popularity and huge sales (especially a movie or play or recording or novel)
a large bomb used to demolish extensive areas (as a city block)