Miki

Miki bot motivates your discord users to stay active. You can set up the leaderboard command and see the most active users and reward them accordingly.
Miki is not just an engagement bot, it can do moderation for your server as well. She can post reminders and ban unwanted users as well.
Everytime a user sends a message, he/she gets some points and you they will be rewarded with virtual currencies. They will be motivated to become the richest user on your discord server.
This bot is completely free to use.