If it his masterpiece then it is his only masterpiece, An artist only gets one unless he goes to another medium. Masterpiece is the piece that signals his mastery of his art, nothing else. He can only do that once. It is probably NOT his best piece because he is still improving his art long after he has created his masterpiece.
In this album, Coltrane was playing hard bop— the major jazz style of the 1950s— and showed he could do it better than anyone. In later albums (I’d single out Giant Steps, My Favorite Things and A Love Supreme), he showed he could move beyond hard bop and point jazz to new directions altogether.