A Steel Mill.
1: Damage would affect the downtime's of FH's, VH,s, BH's & CV's.
2: Not easily taken down. Say each building at Steel Mill Strat has the same Hardness of FH's, VH,s & BH's.
3: Affected downtime of FH's, VH,s, BH's & CV's much lower than objects currently affected by other Strategic targets. Perhaps 1/4.
4: Reduction in Steel Mill Strat's downtime by resupply same as other Strategic Targets.