@final magnet the answer of "best choice" is unfortunately not as simple as it sounds, because the real answer is: it depends.
if you have relational data, consider a relational database (usually SQL) aka "a user has a guild, which has a config, which can have references to other users for permissions"
to get started sqlite is a single-file database, where the module itself does all the transaction management and file locking
once you want to scale up, transferring to another database with a dedicated schedule manager (postgresql, mysqyl (mariadb as an open source fork of that))
note that sqlite is fine for a really, really, really long time, discord bots are usually not that demanding, once you do hundreds of reads per seconds you should definitely consider that
if you have non-relational data aka "i just need one json data object per guild to save settings" (can be more complex than that) a file-based database like mongoDB can be the right choice
(i would personally recommend against quickdb, but that's honestly personal bias with how much issues we've had with them being recommended by really bad bot tutorials online and how they tackled criticism with their method and various errors we have found when looking into it - that might have been remedied already)
at its core, quick.db is a key-value store that wraps around a persistent relational database for more object-like access patterns - i personally think it is worth the fuzz of learning sql or using mongo over this, and if just to broaden your skillset with industry standard approaches