President Reagan is riding high now, but he may have stepped over the line last week when he flouted the State Department and came up with his personal list of the Top Five terrorist nations: Iran, Libya, North Korea, Cuba and Nicaragua.

Until Reagan struck out on his own, the official U.S. list of terrorist nations was considered as inviolable as the Hit Parade or preseason wire-service football polls. This official list, compiled by the State Department, omits Nicaragua and North Korea but includes Syria and South Yemen. Nations had to work to get on the State list or to stay off of it, and no president would have lightly dropped South Yemen to save a few minutes in a speech.

Omission of Syria is understandable. It had helped in freeing hijacked American hostages, which for a nation on the Terrorist List is like losing the big game. This turns out to be a rebuilding year for Syria. No matter what State says, Syria is unlikely to climb back into even the Top Ten during the remainder of the season.

Administration officials, who agreed to discuss the speech on grounds that they be neither identified nor called about late-breaking stories, said centralization of terrorist identification in the White House would never have happened if Alexander M. Haig Jr. were still in charge at State. They acknowledged, however, that State may have "blown it," as they say at the CIA, by dropping Iraq from the Terrorist List shortly before that nation was accused of using poison gas in its war against Iran. "Nobody's perfect," a well-informed official explained in defending the decision.

At a secret dinner meeting on the night Reagan announced his list, some reporters complained that the president was discriminating against other nations that had worked to maintain terrorist credentials.

A friend we will call Huntley, known as a keen student of Eastern European affairs, was particularly upset that Albania was not given even an honorable mention in Reagan's speech. "Here is a country that has worked diligently for 40 years at becoming one of the most unpleasant nations on Earth, and the president ignores it completely," Huntley said.

Poor Huntley. He doesn't understand that Albania has no chance to make any list as long as it limits its terrorism to Albanians. But what of Bulgaria, allegedly involved in the plot of the century to kill the pope? And what of South Africa, which freely terrorizes its own citizens and those of neighboring states?

"Don't be silly," I was told by a reporter still brooding about the slight to South Yemen. "The president likes South Africa."

Other reporters observed that Reagan, perhaps compensating for all his actions against terrorists, had toned down his rhetoric. With the gift of understatement for which he is famed, Reagan merely compared the leaders of these "outlaw states" to Murder Inc., Looney Tunes and Adolf Hitler. "That really helps the cause of fighting terrorism," a well-informed White House speechwriter explained.

What also helps this cause is the sophisticated insight that Reagan brought to analyzing the situation in Nicaragua, his least favorite nation. In a startling discovery, the president revealed that Nicaragua now plays host to Italy's Red Brigades, West Germany's Baader-Meinhof Gang, Spain's Basque terrorists, the Palestine Liberation Organization, the Irish Republican Army and the Tupamaros, a group of Uruguayan guerrillas given amnesty last March. One wonders why this international all-star terrorist cast is whiling away its time in Managua, but it should make Europe safer this summer.

The White House was candid about the significance of Reagan's findings. When Edward P. Djerejian, a spokesman so newly arrived that he still responds on the record, was asked about the omission of Syria and South Yemen, he replied that there is "no definite connection" between the president's list and the official list of terrorist nations. I suspect there is even less of a connection between the president's speech and doing something specific to counter the terrorist menace he so rightly deplores. Reaganism of the Week: Commenting to reporters last Tuesday on the difficulty of rescuing seven kidnaped Americans who remain in Lebanon, the president said: "Part of our problem is we can't answer all your questions on these particular subjects because we can't say what we're going to do next Thursday or there might be someone on the other side that's a little uncomfortable about that."