Oil is measured in gallons, barrels, and tons. This creates confusion when you remember the number but not the unit. One ton = 7.3 barrels = 306 gallons. One barrel = 42 gallons. These numbers are a bit approximate (oil varies in density) but close enough.
The largest spill ever was during the Gulf War and was done on purpose: 11 million barrels. The #14 spill was at Urquiola Spain: 733,000 barrels. Exxon Valdez was only 250,000 barrels and is low in the top 100.
It’s impossible to confidently estimate the size of the present Deepwater Horizon spill, partly because of genuine uncertainty, and partly because information is being withheld while misinformation is being broadcast, but if the upper estimate of 100,000 barrels a day is right, we’re now at 3,000,000 barrels, which would quantitatively be #3 of all time. And this isn’t a “spill”. Spills have an upper limit depending on the capacity of the tanker. We have no idea what the top end of this one is.
The seriousness of a spill isn’t merely quantitative. Biological, economic, and human effects depend on the location (The effects of some earlier spills were also probably underestimated because they were in third world locations, but this is a somewhat different question. ) Land spills spread less far and are probably less damaging than ocean spills.
This particular spill is near a heavily populated area with a huge fishing industry and a lot of tourism, which makes it very serious. I also suspect that a spill in the open sea is less destructive than one in a partially enclosed gulf like this, though I can’t be sure about that. The Gulf of Mexico was already in trouble when this happen, and now it may be ruined for good.
The real lesson here is that the American political system, the American legal system, both political parties, the media, and much of the American public are the slaves of big business and big finance, and big business and finance are legally obligated to be ruthless and conscienceless. We’re going to keep on drilling, the cleanup will be ineffectual, only symbolic and cosmetic changes will be made to drilling practice, and only a few low-level scapegoats will be punished. The people of Alabama, Mississippi, Louisiana, and Texas will defend BP and blame immigrants and Arabs, because what can they do? They need jobs, and employers are watching them closely. BP will change its name, spin off a subsidiary to take the fall and go bankrupt, and walk away unharmed. And that will be the end of it.
Wiki: largest oil spills (Note that the top 14 spills all happened before 1993. We had been making progress.)