Which is the most restrictive isolation level in transaction?

SERIALIZABLE is the highest and most restrictive isolation level in SQL transactions. It prevents all anomalies (dirty reads, non-repeatable reads, phantom reads) by ensuring transactions execute sequentially as if they were running one after another. Snapshot and Repeatable Read allow certain anomalies, while Read Committed allows non-repeatable reads and phantom reads.

Step-by-step Derivation:
Transaction isolation levels from least to most restrictive are: (1) Read Uncommitted - allows all anomalies; (2) Read Committed - prevents dirty reads but allows non-repeatable reads and phantoms; (3) Repeatable Read - prevents dirty reads and non-repeatable reads but allows phantom reads; (4) Snapshot - in some databases, provides MVCC-based isolation between Repeatable Read and Serializable; (5) SERIALIZABLE - the most restrictive, preventing all anomalies by effectively serializing transactions. Therefore, SERIALIZABLE is the most restrictive isolation level.