Purely from observation throughout the years the most popular seems to be summoners war, people tend to not talk about it as much these days but I know the player base is still huge.

On this sub Reddit however the games that constantly get bought up would be Azur Lane, Granblue Fantasy and Fate Grand Order.

If you want to pick a game based off an active community I suggest you check out the stickied thread on front page as the Mod has kindly made a table outlining the games with the largest subreddit community.

Niche games are not often suggested due to their unique play style but are very popular in their own right such as BanG Dream and Love Live if you enjoy rhythm games, One Piece Treasure Cruise if you like tap timing games or even Chain Strike for its unique chess-like gameplay.