Why is the security council the most powerful arm of the united nations (un)?

The Security Council of the United Nations is responsible for the maintenance of international peace and security.  It is the only group or agency within the UN that can make decisions which are binding on all members of the United Nations.

The Security Council has the authority to send peacekeeping forces into conflict zones and to impose sanctions on member states of the UN that are deemed to be acting against the cause of peace.

The Security Council also approves new member states of the UN and plays a role in the election of judges to the International Court of Justice.