Both are equally correct. 'That' is optional when it represents the object of the relative clause: we'll play the song tonight. It is required if it represents the subject: This song brought us together, so this is the song that brought us together.
Both are equally correct. 'That' is optional when it represents the object of the relative clause: we'll play the song tonight. It is required if it represents the subject: This song brought us together, so this is the song that brought us together.