Solid wood or metal doors, long screws in the sills and hinges that make it into actual studs, metal plating to reinforce the hinges as well as the edge of the door on the lock side (an extended strike plate), and maybe even something like these door bars or floor based door jams
Also consider reinforcing any walls or windows around a door. Windows can be reinforced with window bars or a security film. Walls will need to have some type of security mesh installed under the drywall.
Here is a past post I made about reinforcing entryway doors.
And here is one I made about reinforcing bedroom doors