Abidemi Kusimo

You can make use of category popular tag to display most used tag on your blog. This will give list of frequently used tag anywhere you place the shortcode or funtion in your theme.
For example [popular_category_tags count=10 type=”popular-tags” category_id=”1″]. 
You can also call this function from your theme file: cush_category_popular_tag() for basic output.
When using the shortcode type is required. category id, count is not required. if no category id suplied, the plugin will default to current post category available. 
For example [popular_category_tags type=”popular-tags” ].
You can find category tags options under Settings page 
Settings -> Category Tags
Credit : (https://wordpress.stackexchange.com/questions/261617/display-most-popular-tags-of-category)

Category Popular Tags

Display popular tags on achieve/category page of your theme using sortcode or by calling a function.

gdthemes

A very simple popular posts widget which displays posts by comment count (with featured image).
This is a very simple plugin which displays your posts by comment count when activated at your desired sidebar. 
It comes with a setting whether you’d like to display your post’s featured image or not and also be to adjust the width and height of it.
No CSS styling has been done and it is up to you to customize it.
Please write a review or rate if you like it! 🙂
<h3>Upgrade</h3>
<ol>
<li>Use the plugin updater in WordPress or…</li>
<li>Delete the previous <code>my-popular-posts-widget</code> folder from the <code>/wp-content/plugins/</code> directory</li>
<li>Upload the new <code>my-popular-posts-widget</code> folder to the <code>/wp-content/plugins/</code> directory</li>
</ol>
<h3>Usage</h3>
<ol>
<li>Just install and activate.</li>
</ol>

My Popular Posts Widget

A very simple popular posts widget which displays posts by comment count (with featured image).

Category Popular Tags vs My Popular Posts Widget