Cloud SQL is the safe default — every other row needs a named reason.
The first fork narrows the field before scale even matters.
Relational, generalCloud SQL
Perf ceiling hitAlloyDB
Global + strong consistencySpanner
Document / wide-column / analyticsFirestore / Bigtable / BigQuery
Never choose Spanner or AlloyDB because they sound more advanced — both carry real cost and complexity, justified only by a specific, measured requirement.