The general thumb rule is to import all master transactions. The sequence is always all master transactions which are not dependent on other masters to be imported first and then subsequent masters. Once all masters are imported then related transactions can be imported to reference the masters and import. NetSuite provides a great CSV tool to import both masters and transactions. For further complex data migration needs you can use webservices and write your own connector as well.
NetSuite suggests to migrate data in the following sequence:
- Chart of Accounts
- Communication & Activities
- Relationships/ Entities
- Opening Balances