What kind of game is it? Requirements for a MMO will be different to say, a shooter, or something with a leaderboard.
Firebase is usually the default. It's not as simple as it looks and can be a little costly, but it's well supported.
If it's just CRUD, Parse Server is much faster to set up and cheaper. It's was open sourced by FB and suddenly everyone lost interest as it was no longer a resume item.
If you want something more customized, less simple, but with a little latency, serverless stuff is fine.
