From Popular Mechanics.

Exxon Valdez, at about 11 million gallons, doesn't even make the list.

The USGS estimates the Deepwater Horizon leak rate at an upper range of about 800,000 gallons a day. At that rate, assuming nothing slows it down, Deepwater Horizon will crack number 10 on the list on about June 12. To match Ixtoc, the leak would have to run unabated about six months. It would take about 300 days to match the amount of oil Saddam Hussein dumped into the Persian Gulf.

From the article, an assessment of the Gulf War spill:

^The largest oil spill the world has seen exacted little permanent damage on coral ecosystems and local fisheries, according to a report by the Intergovernmental Oceanographic Commission at Unesco. The study concluded that about half the oil evaporated, one-eighth of it was recovered and another quarter washed ashore, mostly in Saudi Arabia.^