Endgame.
Opening vs Endgame
Openings, but not really deeply. Just the basic principles and creating a basic opening repertoire for both white and black (not very deep, just few moves). Then move on to study endgame thoroughly (first pawn endgames, then rook, bishop, etc )
End game of course. It is simpler less men. It can be taught exactly. Openings are more fashion and opinion, no exact knowledge.
Which one to study first? Opening vs Endgame