Thanks to the contributors, there's so many powerful plugins in C2 forum. However, they're not organized. Sometimes it's hard to find and choose right plugins for your project. They could be unpopular, or not working for your platform. So I want to start this thread with sharing my small experience.
Local Leaderboard
https://www.scirra.com/forum/plugin-local-leaderboards_t85519?&hilit=local+leaderboard
This plugin manage your highscores in its own Webstorage. You can call the values by looping. You don't have to compare every score each other to make offline leaderboard. Works in CocoonJS and Crosswalk.
Move to
https://www.scirra.com/forum/behavior-moveto_t63156
It's quite a famous plugin and really useful for the beginners. Moves objects from one point to another with various actions. Works in CocoonJS and Crosswalk. If there's a list like "Basic plugins useful for beginners", this plugin must be included.