Among the free to play games that have been maintained the longest is Clash of Clans by Supercell. And its theme of assaults, wars between clans of barbarians that combines magical creatures, potions and mythological beings with fun. Thanks to its nice animation, the competition between clans and all the features that make it so addictive has managed to get more than 568 million active players.
Understand that Active Players is not the same as created accounts. If the numbers of accounts created and then abandoned were given, it would be incredible, given how many years the game has been and the popularity in its early years when it did not have as much competition.
Recall that even the legendary actor Liam Neeson advertised this game, which resulted in a lot of new players.