The strictest transaction isolation level provided by SQL Server is?

The strictest transaction isolation level provided by SQL Server, and by the SQL standard, is SERIALIZABLE. This isolation level ensures that transactions are completely isolated from each other, effectively preventing all common concurrency anomalies, including dirty reads, non-repeatable reads, and phantom reads. It achieves this by ensuring that any concurrent execution of a set of Serializable transactions produces the same effect as if those transactions were run in some serial order, one after another.