It said the main hospital and train station had been destroyed, along with thousands of other buildings.
Local officials told the UN team 64 bridges and much of the electricity grid had been ruined.
Iraqi forces declared victory over the jihadist group in Ramadi in December and has since cleared most of the western Iraqi city.
IS fighters still hold pockets in the northern and eastern outskirts.
Its recovery boosted prime minister Haider al-Abadi in his campaign to oust the militants from their northern stronghold of Mosul later this year.
However, more than six months of fighting shattered most infrastructure and levelled many homes in Ramadi, the Anbar provincial capital where around half a million people once lived.
The fighting saw IS bomb attacks and devastating US-led coalition air strikes.
“The destruction the team has found in Ramadi is worse than any other part of Iraq. It is staggering,” said Lise Grande, the UN’s humanitarian co-ordinator in Iraq.
The two-day assessment found nearly every building had been damaged or destroyed in frontline areas.