Yes, MEGAHITS is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, G 2, A 1, H 4, I 1, T 1, S 1.

Is it a valid word?

MEGAHITS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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

Also: smash hit.