For a prototype with unpredictable usage and global users, the best free serverless databases are usually the ones that give you:

• auto-scaling / pay-as-you-go
• simple setup
• global replication or low-latency access
• a generous free tier
• good integration with serverless apps

Here are the strongest options:

1) MongoDB Atlas Serverless

Best for: flexible document data, fast prototyping

Why it’s good:

• Fully managed and serverless-style scaling
• Easy to start with a free/shared tier
• Great if your data is semi-structured or changing often
• Widely supported by serverless frameworks

Tradeoffs:

• Not ideal if you need strict relational transactions
• Global distribution isn’t as seamless as some multi-region-first options

2) Firebase Firestore

Best for: mobile/web apps, realtime sync, simple document models

Why it’s good:

• Very easy to use
• Serverless and scales automatically
• Excellent SDK support for web/mobile
• Good for prototypes that may become apps quickly
• Built-in realtime updates

Tradeoffs:

• Query model can be limiting
• Costs can grow unexpectedly with lots of reads
• Not a traditional SQL database

3) Supabase Postgres

Best for: prototypes that need SQL, joins, and a relational model

Why it’s good:

• Free tier is very developer-friendly
• PostgreSQL is familiar and powerful
• Great if you want SQL from day one
• Easy auth/storage integration

Tradeoffs:

• Not as globally distributed “out of the box” as some others
• Free tier is not meant for heavy unpredictable load
• More of a managed backend than pure serverless DB, though it works well for prototypes

4) Neon Postgres

Best for: serverless SQL with branching, flexible dev workflows

Why it’s good:

• Serverless Postgres
• Very good for prototypes
• Branching is excellent for testing and staging
• Scales down when idle

Tradeoffs:

• Global user performance depends on your app architecture
• Free tier is great, but production usage may need careful planning

5) Upstash (Redis / Kafka / Vector / QStash)

Best for: lightweight data, caching, queues, rate limiting

Why it’s good:

• Truly serverless feel
• Great free tier for prototyping
• Useful for global, low-latency access
• Very convenient for edge/serverless apps

Tradeoffs:

• Redis is not a primary database for most apps
• Best used as cache, session store, queue, or lightweight state layer

6) DynamoDB (AWS)

Best for: extreme scalability and unpredictable traffic

Why it’s good:

• Highly scalable, serverless, and robust
• Good free tier
• Handles unpredictable usage very well
• Great if you’re already on AWS

Tradeoffs:

• Data modeling is more complex
• Query patterns must be designed carefully
• Less friendly for quick prototypes unless you’re comfortable with NoSQL design

7) PlanetScale

Best for: MySQL-based prototypes needing strong scaling characteristics

Why it’s good:

• Excellent developer experience
• Branching workflows
• Scales well
• Good for prototypes that may become serious products

Tradeoffs:

• Less “serverless database” in the strict sense than some others
• Free tier and feature set have changed over time, so check current availability
• Global low-latency is not as automatic as edge-native systems

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Best picks by use case

If you want the easiest prototype

• Firestore
• Supabase

If you want SQL

• Neon
• Supabase
• PlanetScale (if MySQL is fine)

If you want global scale + unpredictable traffic

• DynamoDB
• Firestore
• MongoDB Atlas Serverless

If you want simple serverless app primitives

• Upstash
• Firestore
• Supabase

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My practical recommendation

If you’re building a prototype with global users and uncertain load, I’d narrow it to:

Top 3

1. Firebase Firestore — easiest and very good for global app prototypes
2. Neon Postgres — best if you want SQL and a “real database” feel
3. MongoDB Atlas Serverless — good flexible middle ground

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Important caution

“Free” serverless databases are great for prototyping, but with unpredictable usage, watch out for:

• read-heavy billing surprises
• cold starts / latency
• regional performance
• free tier limits
• egress/network costs

If you want, I can also give you a ranked comparison table based on:

• free tier generosity
• global latency
• SQL vs NoSQL
• ease of use
• best for prototypes