Recently I had a project in WordPress that has main categories, subcategories, and the content.

Main Category
|
|_ Subcategory Content
|
|_ The Content

Here’s how to display the parent category. Parent Category of The Content will be Subcategory Content, and Subcategory Content’s parent will be Main Category.

if($post->post_parent) {
    $parent_title = get_the_title($post->post_parent);
    echo $parent_title;
}

And here’s how to display Main Category title from all pages whether you’re in Subcategory Content or The Content.

if ( 0 == $post->post_parent ) {
    the_title();
} else {
    $parents = get_post_ancestors( $post->ID );
    echo apply_filters( "the_title", get_the_title( end ( $parents ) ) );
}