The NGOs I worked for weren't buying brand new 4x4s and reinforcing expensive compounds like the UN do.
Neither did they send peacekeeping troops to Africa and Asia where they started spreading the AIDS virus, and subsequently covered it up for five years without lifting a finger, unlike the UN.
Cambodia owes its AIDS problem almost exclusively to the UN because of this.
Nor did they make billions in profit from the oil for food scandals, helping Al Qaeda and senior figures in the Taliban make stupid amounts of money via Dubai's oil companies.... as well as ensuring the Swiss, American, British and French banks were well and truly fattened whilst the people of Iraq were starving under Saddam.
Sure, every company has it's share of corruption, and no NGO will ever be completely free from it, but there is no comparison there with the sheer scale of damage done by the United Nations.
This guy sums it up for me...
http://www.cato.org/pubs/pas/pa-253es.html