This app was mentioned in 2 comments, with an average of 1.50 upvotes
I think the most succesful, True-To-Term mobile mmo is Iruna Online. It serves as the main income pillar for Asobimo, who has several other mmo's running with different mechanics/themes to each of them.
Link to Iruna on the Play store: https://play.google.com/store/apps/details?id=com.asobimo.iruna_en&hl=es_419
My favorite from Asobimo has to be Stellacept, because it reminds me a lot of Phantasy Star due to the aestetics. Link to Stellacept: https://play.google.com/store/apps/details?id=com.asobimo.stellacept_online_en&hl=es_419
The longest running MMO has to be Order&Chaos, and counts with a new installment called Order&Chaos 2. I don't suggest touching either with even a ten inch pole.