what is the right data type for unique key in postgresql DB?

bigint (or bigserial if you need auto-incrementing keys) is just fine.

If know for certain that you are not going to load too many rows, you might consider integer (or a regular serial) and potentially save some harddisk space.

Use the serial type for automatically incrementing unique ids.

If you plan to have more than two billion entries, use bigserial. serial is the PostgresSQL equivalent of MySQL's AUTO_INCREMENT.

PostgresSQL Documentation: Numeric Types