It's tough to say, but I hypothesize it was WWII.
What a great question!
This is a very hard question to answer, because ecological data before 1960 is sparse and difficult to get. This is particularly true during World War II, where we must work with after-the-fact data by and large. Environmental history is also a relatively new field, which means there's a lot of unexplored ground. Natural Enemy, Natural Ally was published in 2004 and was really one of the first big books to look at the history of war's impact on the environment.
So not only are you looking at a narrow historical niche (environmental history), you're looking at a subsection of that niche (environmental history of war) and a sliver of that subsection (World War II.) Simo Laakkonen is one of the people working on a global environmental history of World War II, which hopefully will be out soon. Last year, he was giving a bunch of talks on WWII environmental history while he was the visiting lecturer at the University of Kansas. I haven't heard anything about the book he was editing, and a quick search didn't turn up anything.
Anyway, let's get on to your topic by trying to define "worse." If you look at simple statistics, the actions of World War II win by a long shot. The Exxon Valdez had a listed capacity of 1.48 million barrels. At the time of its 1989 spill, it was carrying 1,264,155 barrels of Alaska North Slope crude oil. According to the Exxon Valdez Oil Spill Trustee Council, about 257,000 barrels spilled in the disaster.
The Deepwater Horizon disaster far surpassed that total, leaking an estimated 4.9 million barrels of Gulf of Mexico crude oil. BP thinks that figure is about 800,000 barrels too large (the company did capture some oil before the slick reached the surface), but in either case, Deepwater Horizon is by far the largest single crude oil spill in U.S. waters.
Now, let's look at the effect of World War II. The Pacific Regional Environment Programme has a database of over 3,800 ships sunk in Asia-Pacific waters during World War II. Of these, 330 are tankers and oilers. The Atlantic, Mediterranean and Indian Ocean WWII shipwreck database contains more than 4,000 sunken vessels over 1,000 tons, and 529 of those are oil tankers. The AMIO database was not complete as of this 2004 paper, but I believe the SPREP database is.
In any case, you're looking at about 860 known oil tanker sinkings during the war. That's a huge figure, but you need to add some qualifications.
Of the 529 tankers in the AMIO database, 105 are known to have been empty (in ballast) when they were sunk. Furthermore, a tanker carrying gasoline, diesel or aviation fuel is much less of an environmental hazard than one carrying fuel oil or crude oil. The former petroleum products evaporate quickly, and generally not as much effort is made to clean them up if they spill. The only exception is if they're right on shore or close to a particularly sensitive area. Fuel oil and crude oil are heavier. They linger and can cause long-term environmental damage.
Furthermore, not all types of crude oil are equal. Alaska North Slope Crude is much heavier in general than Gulf of Mexico crude oil. Refiners and drillers talk about "light, sweet crude" when they refer to a type of particularly valuable oil. There are other names for other types.
In World War II, most tanker sinkings (particularly after 1942) involved refined petroleum products. Don't think tankers hauling crude to refineries — think tankers hauling gasoline from the U.S. to the UK. Think of the embattled convoys hauling supplies to Malta or accompanying fleets at sea. They're carrying bunker oil, not crude.
There's another factor that comes into play when you talk about environmental damage. It's an old saying: "Dilution is the solution to pollution."
That saying comes from the fact that environmental damage (or sickness) typically comes from a specific concentration of a pollutant in the environment, not from the presence of that pollutant. If you have one part per billion of crude oil in water, you're not going to have appreciable environmental damage. Cover the surface of a lake with the stuff, and you're going to have big problems.
If you sink a ship far out at sea, it's going to cause far fewer problems than if it's sunk in a port or close inshore. That's why the environmental damage from the Exxon Valdez was so much greater than that of Deepwater Horizon, even though the latter spill was larger. The Exxon Valdez accident (in addition to spilling thicker oil) happened inshore, and not only that, it took place in semi-enclosed Prince William Sound. Deepwater Horizon took place in the open ocean, where oil had to travel 45 miles before coming ashore.
The tankers sunk during the war were all over the place, with high densities along the U.S. East Coast, the Caribbean, the Vietnamese coast, and in the waters between the Philippines and Taiwan.
Let me conclude with one small fact: The environmental damage done by sunken WWII tankers is not over.
In August 2001, the Federated States of Micronesia declared an emergency in Yap state following a massive oil spill. The culprit was the USS Mississinewa, which sank on Nov. 20, 1944 after it was destroyed by a Japanese suicide submarine. The tanker was loaded with 146,000 barrels of aviation gasoline when it was ripped apart by a series of violent explosions caused by the Japanese attack.
Despite the violence of the sinking (which killed 63 people), much of the Mississinewa's cargo remained aboard the sunken ship. In 2001, a typhoon passed over the Yap atoll, tearing apart the corroded hulk of the Mississinewa and releasing thousands of gallons of fuel oil. The U.S. Navy responded by sending divers to plug up the sunken ship's hull, but more than 5 million gallons of oil is believed to remain aboard the sunken tanker.
This ship is one of hundreds of similar cases. We have not seen the last one.