President Reagan heads for his summit meeting with Soviet leader Mikhail Gorbachev next week having revived public confidence in American foreign policy but without a towering achievement to leave as the centerpiece of his presidential legacy, according to a wide spectrum of admirers and critics alike.

Despite enduring popularity and a trillion-dollar defense buildup, the president has yet to parlay these assets into a landmark of diplomacy comparable with the 1977 Camp David accords of President Jimmy Carter or the reopening of U.S. relations with China engineered by President Richard M. Nixon.

Instead, the administration's foreign policy has suffered several notable setbacks in the last two years: the humiliating withdrawal of U.S. peacekeeping troops from Lebanon after the loss of 241 American lives; the collapse of Reagan's "constructive engagement" policy in southern Africa; the imposition of sanctions against South Africa, contrary to the president's wishes; the congressional refusal to sell arms to Jordan, and the teetering of the Australian-New Zealand-United States (ANZUS) alliance.

Furthermore, other unresolved problems threatening U.S. interests -- such as the fate of the Marcos regime in the Philippines and the Third World debt burden -- are ticking like bombs waiting to explode in the second term of the Reagan administration.

How these past failures and frustrations weigh on Reagan's aspirations for a successful summit remains unclear. Nonetheless, many analysts believe the meetings Tuesday and Wednesday could mark the start of a new phase in the administration's foreign policy, in which East-West relations will dominate and which could determine the president's legacy as a statesman.

Moreover, perceived failure at the summit could have a marked impact on public opinion. Reagan came to office promising that he knew better how to deal with the Soviet Union than his predecessors, and this meeting in Geneva may well be regarded by the public as the ultimate test of his ability to redeem that promise.

Reagan's fiercest critics, such as Harvard University Professor Stanley Hoffmann, assert that Reagan has shown little interest in foreign policy and has simply "staggered from issue to issue and place to place" with no comprehensive strategy.

"No major disasters and no smashing successes," said Hoffmann, summing up Reagan's five years in office.

But the president's supporters sharply disagree. They credit Reagan with several bold initiatives, including his Strategic Defense Initiative, which the president believes could fundamentally alter the basis of both superpowers' nuclear strategy by deemphasizing offensive arms and concentrating on defense.

They also applaud what they see as a steady course in foreign policy under Reagan, compared with the Carter-era zigzagging. They cite his avoidance of any protracted Iranian-style debacles in Lebanon and elsewhere.

In addition they note Reagan's success in fostering a moderate government in El Salvador, which has had considerable military success against leftist rebels, in large measure because of U.S. support.

Critics and admirers also agree that Reagan has enjoyed both good luck and extraordinary public tolerance for his blunders.

Nevertheless, even Reagan's admirers admit it is not easy now to assess how history will judge him.

"It's hard to say what final grades he will get," said William G. Hyland, a former aide to ex-secretary of state Henry A. Kissinger and now editor of Foreign Affairs.

"The big problem is how to cash in on what he accomplished in the first term."

Even a former Carter administration official like Charles W. Maynes, editor of Foreign Policy, is impressed by Reagan's success in demonstrating that the United States can "throw its weight around in the world," using force in Lebanon, Syria, Libya and Grenada, pulling out of UNESCO and limiting U.S. participation in the World Court "with fewer repercussions than people thought [likely] in the mid-1970s."

"The fact is," Maynes added, "there hasn't been the degree of resistance or diplomatic cost one might have anticipated."

Perhaps Reagan's biggest accomplishment, according to Hyland and others, is that he has dispelled the "national malaise" created by the long, debilitating Vietnam war and the Iranian hostage crisis that ended the day he took office in January 1981.

"He has restored a sense of national self-confidence and restored it with key allies," Hyland said. He rated Reagan's success in getting key NATO allies to take American Pershing II and cruise missiles, despite heavy Soviet pressure, as probably his most significant achievement in East-West relations.

James R. Schlesinger, who has served in both Republican and Democratic administrations but is often critical of Reagan defense policies, also regards a renewed American self-confidence as the president's chief accomplishment. The era of public "self-flagellation" about U.S. policies abroad that prevailed during the 1960s and 1970s is over, he said.

At home, polls indicate that public confidence is growing in Reagan's handling of foreign affairs. A recent Washington Post-ABC News poll showed Americans giving him the highest approval ratings of his presidency on diplomacy issues.

"There is a general thought that the president comes into this summit in November in very strong domestic political shape with probably as high approval for foreign policy as he has had in any stretch," said Sen. Richard G. Lugar (R-Ind.), chairman of the Senate Foreign Relations Committee. "There's a feeling that the general course that he has prescribed has worked fairly well for us."

To a degree unanticipated by his critics, domestic considerations and political pragmatism appear to shape Reagan's diplomatic course rather than the kind of conservative ideology exemplified by his description of the Soviet Union as an "evil empire."

This flexible foreign policy rudder is credited by some analysts as preventing the administration from becoming mired in such potential bogs as Lebanon and Nicaragua, where Reagan's initial actions suggested a headlong plunge into local politics without adequate congressional or public support.

Probably the most contentious issue in the assessments of Reagan's performance abroad concerns his inconsistency in pursuing his announced policies.

Abrupt shifts in course, exemplified by the Lebanon pullout and his attitude toward the Soviet Union and the value of summitry, have rankled both conservatives and liberals.

Critics such as Hoffmann vehemently dispute Hyland's praise of Reagan for "holding a steady course" and perceive a zigzagging in his policies, or at least a discrepancy between word and deed reminiscent of Carter.

Hoffmann attributes this to a "remarkably divided administration," despite "an apparently unified world view," in which "hard-liners and harder-liners" have battled over issues ranging from the Soviet Union to southern Africa.

"I don't see any real steadiness," he remarked. "He has given the impression of it but not the reality of it."

Hoffmann notes the paradox of a Reagan who has neither lived up to the expectations of his most conservative cold warrior backers nor proven to be the gun-slinging "pyromaniac" liberals feared.

Reagan had been faulted for failing to support his tough rhetoric on terrorism with tough action. After promising "swift and effective retribution" against terrorist acts, he seemed at a loss for years until Oct. 10 and the stunning interception by U.S. warplanes of an Egyptian airliner carrying four hijackers of the cruise liner Achille Lauro.

Still plaguing the administration, however, is the lack of success in freeing the remaining six Americans being held by Shiite terrorists in Lebanon for more than a year.

On some issues, such as implementation of the so-called "Reagan Doctrine" aimed at rolling back Soviet domination in the Third World by aiding pro-Western "freedom fighters," Reagan has puzzled his conservative backers by dining at the White House with the Marxist leader of Mozambique, Samora Machel, while refusing aid, so far, to rebels fighting the Marxist Angolan government.

"I think there is still a pragmatic search in each case as to how the implementation [of the Reagan Doctrine] goes," Lugar remarked. "The jury is still out in specific cases as to how well it will work."

That same "jury is out" judgment is applied by Reagan's supporters and critics to many other aspects of his foreign policy. There is near-unanimous agreement that the toughest tests lie ahead.

Despite the summit and resumption of arms control talks -- and the suggestion of a new administration preoccupation with East-West issues -- many observers believe the most troublesome crises are likely to be rooted in the Third World.

"The Soviet relationship is the most important for us," Lugar said. "But I think the potential difficulties for us, foreign policy-wise, in the next year or two lie much more in our inability to manage debt with friends in our own hemisphere."

Noting the return of democracy in half a dozen Latin American countries and "fledgling democracies" now in El Salvador and Honduras, Lugar warned that new protectionist measures or higher interest rates here risked reversing this favorable trend.

"We could pretty well ruin the economies of most of the governments in our hemisphere and jeopardize the building of democratic institutions which is so important for our future and theirs," he said. The result would be to "undermine anything that we thought we had done in a tactical military way" to contain the spread of communism in Central America.

Meanwhile, tough policy decisions, with the potential for "a series of tragic blunders," as Maynes put it, await the Reagan administration in its handling of the crises in the Philippines and South Africa, the delicate Middle East peace process and the application of the Reagan Doctrine to other countries, such as war-torn Angola.