hen the supertanker Amoco Cadiz went aground off the coast of France four years ago, it created what was then the largest oil spill in history.

It also has created what some lawyers are calling the "admiralty case of the century" now under way in a crowded Chicago courtroom. Attorneys for the owners of the ill-fated supertanker, on the other hand, dismiss it as the "most overrated case of the century."

The nationalities involved in the dispute would seem to qualify the case for United Nations' arbitration.

The American-owned Amoco Cadiz, three and one-half football fields in length, carried the flag of Liberia when it broke up on a reef two miles from the coast of Brittany. It spilled an estimated 58 million tons of Arabian crude oil that coated 200 miles of the French coastline and, reportedly, Britain's Channel Isles as well. Its captain was Italian, as was most of its 40-man crew. The tug involved and her crew came from Germany. And the ship herself was built in the same Cadiz, Spain, shipyard that spawned the Pinta, Nina and the Santa Maria.

There has so far been no attempt to resolve the amount of damage caused by the spill. The first phase of the trial, which began May 4 before U.S. District Court Judge Frank J. McGarr, is to determine where the blame for the wreck lies. Only then would the issue of damages arise.

On the right in McGarr's courtroom sit attorney Frank Cicero and an assistant or two, representing the major defendants in the case, Standard Oil Co. of Indiana and its subsidiaries, Amoco Transport Co. and Amoco International Oil Co.

On the left side of the courtroom sit about a dozen attorneys representing the plaintiffs: the Republic of France and 50 of Brittany's local governments; 36 hotel owners on the Cote du Nord; the tug company, Bugsier, Reederei und Bergungs AG; Shell Oil Co., which owned the crude; and its insurer, Petroleum Insurance Ltd.

They charge that Amoco Transport was negligent in the maintenance of the ship's steering system, and that the company allegedly knew the system was defective before the steering gear failed, causing the 200,000-ton vessel to drift toward the shore.

Standard of Indiana and its subsidiaries also are charged with inadequately training the officers and crew, deferring repairs to increase profitability, and delaying help from the tug Pacific by haggling over the cost of the rescue effort.

Cicero has filed countersuits against the French government and Bugsier, saying both were negligent in failing to salvage the Amoco Cadiz.

The "admiralty case of the century"--so-called by the hotel owners' attorney, John J. Kennelly of Chicago--is being conducted 750 miles from the nearest ocean partly because Standard of Indiana is based in Chicago. Cicero says the federal court here became a logical place to consolidate several suits filed in New York City, Norfolk, and Chicago.

At the same time overseas, the ship's master, Captain Pasquale Bardari, has been facing French criminal charges, the United Kingdom is filing for a $4 million chunk of a fund Amoco International set up in Brest, France, after the tanker's breakup, and Amoco International and Bugsier are submitting to arbitration in London.

Kennelly's characterization isn't shared by all sides, clearly. "It certainly isn't the most complicated case I've ever tried, not in terms of the facts and the nature of the facts," Cicero said.

"This is the most overrated case of the century," said John C. Smith, who represents Petroleum Insurance Ltd. No one is making confident estimates on the duration of the case. One lawyer said he expects a monetary verdict to be issued by the summer of 1983, but he isn't making any guesses about how long appeals might take.