megahit vs smash hit

megahit and smash hit both are nouns.

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

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

megahit 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 megahit include blockbuster.

megahit (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)