Discord is the best currently, RocketChat is better for more advanced deployments and if you have the money available to host it. Better than Slack and Discord if you have the time to configure it as well, but not completely user friendly like Discord is yet.

Join by clicking Chat in the Upper Menu. Or this link:

Join the UtopiaEducators.com Discord Server!
Check out the UtopiaEducators.com community on Discord - hang out with 91 other members and enjoy free voice and text chat.