Medito is one of the best app for meditation. Medito is completely free and the features it offers for free just makes this app best on the list.
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Insight Timer has decent features on free plan, you can get additional features in premium plan. You can see how many people were meditating with you.
CALM isn't free but it offers great features for free as well like guided meditation and sleep story. Try their 7 day free trial before deciding to subscribe
I have tried all of these apps ATOM and MEDITO are the best according to my needs. I have written article on 4 of these free apps with all their features