The big name weapons like Brahmastra are often neutralized by the same or similar weapon. When was the most damage inflicted by a single use of a weapon?
Indrajit employed Brahmastra by which Sixty seven crores of Vanaras were struck down in a single day.
सप्तषष्टिर्हताः कोट्यो वानराणां तरस्विनाम् | अह्नः पञ्चमशेषेण वल्लभेन स्वयम्भुवः || (Yuddha Kanda 74 Sarga 12 Sloka)
Sixty seven crores of powerful monkeys were struck down by the cherished missile of Brahma, the self-born creator, in the fifth and last period of the day.