What is the best way to insert 6000000 records from one table to another table in ORACLE?

To expand a bit on Anders' answer and mathguy's comments, do the following:

alter table data nologging;
alter session enable parallel dml;
-- disable any triggers on `data` and temporarily drop any indexes

insert /*+ append */ * into data 
select /*+ parallel (4) */ * from tmp_data
--sample (10)  -- if tmp_data has 60 million rows: 10 means 10%
-- where rownum < 6000001 
-- pick one of the two prior clauses if tmp_table has > 6 million rows

after the insert is done:

alter table data nologging;  
-- enable triggers and recreate indexes

and have the dba do a backup as the data table will not be able to be recovered if there was any issue after the load.