Energy...no brainer
If you saw the interview she did with Jake Tapper early last year when she met with Trump before he announced, Jake asked her about what she would like to do. She immediately said Energy but also quantified that in would be the shortest post in history because she would close it down almost immediately.
My take even then was to Take Interior and in such start to migrate virtually everything into it and then cull the forest of dead wood. That would include Energy, EPA, Transportation, TSA, DHS and so forth. Security goes back to the CIA and FBI where it belongs.