Other people will suggest SQLite and Postgres, so I'll go for something less conventional... If you are using GCP or AWS, take a look at Firestore and Dynamo.

Other people will suggest SQLite and Postgres, so I'll go for something less conventional... If you are using GCP or AWS, take a look at Firestore and Dynamo. They are fully managed (no maintenance for you), reliable, fast, cheap, scalable, and easy to use. Despite being designed for heavy-duty usage, they work perfectly as a simple key-value store. I use Firestore for all my personal projects, and I only pay a couple dollars per month.