Steel doors are the strongest. Although hardwood doors are sturdy, they are easier to break into than steel units, which do not warp or crack. Realistically, the only damage a steel door might suffer is a dent, perhaps a few scratches. It would take an acetylene blow torch or an angle grinder with a metal-cutting disc to cut through steel, which would be highly impractical, time-consuming, expensive, and super risky for a criminal to attempt.