Missil interceptor batteries. They are most efficient by far when factoring in the cost of buying+staffing them. You need about 17250 of them to get to level 2 (75%) assuming you have only one country. (In practice, you will likely need less, since you already have some defence. For a rough estimate (not exact since it is a rolling average), take the % difference between your current level, and 75, and multiply this number by 230. That is how many missil intercepter batteries you need.)