The best way to fetch the best class within the hierachry for a selected submit?

I perceive WordPress idea “major class”, however I don’t perceive easy methods to fetch the identify of the best class within the hierarchy for a selected submit?

Having this code:

$category_obj = get_the_category( $post_id );
$category_name = $category_obj[0]->cat_name;                   

would fetch the class’s identify of the submit, however this class is simply the primary class listed for the submit and that class shouldn’t be all the time the first class.

I am utilizing this outdoors of “the loop”.