I think this plugin has a ton of potential for user engagement but it doesn't use as well as it could.
For example, if there was an option to display more than 1 post, it'd be a lot more useful already.
If there was a way to include posts from a specific category (i.e. 'featured'), it would be crazy useful.
Finally, you should add a custom action hook so that we can define our own functionality (I can easily do that with do_action() but please consider doing it in your plugin) using the best method WP already provides for us. This way, I can add the popular posts using any plugin I want without touching your plugin source at all.