Imagine, for a moment, a magnificent and colorful contest to determine the world's best flavors of ice cream. Suddenly chocolate, strawberry, peach, pistachio, maple walnut and a handful of others withdraw.

Vanilla, coffee, chocolate chip, butter pecan and a bunch of ripples insist that the contest will go on regardless. They are indisputably good, and might have won the ultimate taste test against all the others, but who can say for sure? Much of the significance of the competition would melt away.

As a sporting event, a Summer Olympics without the United States and its major allies would be similarly devalued -- interesting and flavorful for some connoisseurs, but far from definitive or compelling to the general public.

Any sports festival taken seriously by the Soviet Union, East Germany, the rest of Eastern Europe, Cuba and a substantial part of the Third World must be significant -- much more so, for instance, than last summer's Spartakiade, the Soviet sportfest to which most Eastern and Western sports powers sent only token teams.

Conducted under the five interlocking rings emblematic of the Olympic Games, an earnest Soviet Bloc-Third World meet would have to count for something. But many sports would conspicuously lack the East-West rivalry and multinational depth that give the Olympics credibility as the world's gold standard in amateur atheletics.

An Olympic basketball tournament without the United States, for example, is hardly worth the name.

Men's gymnastics without the Japanese and Chinese is less than Olympian in stature.

Yachting without Australia, Great Britain, the Scandinavian countries and the U.S. is clearly off-keel.

Equestrian events conducted without the British, Irish, Americans, Swiss and West Germans could be considered world class only by the rear end of a horse.

Such are the prospects for this summer's Olympics in Moscow if a wide-spread boycott develops, as now appears likely, to repudiate the Soviet Union for its invasion of Afghanistan. The Moscow Games could be reduced to a second-or third-rate international competition in most of the sports Americans care about, and a few they don't.

Now that the U.S. Olympic Committee has decided not to enter the Moscow Games, West Germany, Japan and Canada seem certain to follow suit. Australia and much of Western Europe are likely to join the boycott.

Several sports federations in Great Britain, including those governing yachting and equestrain events, have decided not to go to Moscow. Other British sports will be under pressure to reverse their decision to go if Britain's Common Market partners decide to stay home.

China, which finally won a 30-year intramural contest with Taiwan and was readmitted to the Olympics last Novemver, was one of the first countries to join the boycott.

Kenya, which has several potential medalists in track and field, including the formidable Henry Rono, has indicated its intention to boycott Moscow, and some other black African nations may follow.

Most surveys of the prospective boycott have focused on its political aspects, listing the number of governments and Olympic committees for and against as if they were lining up for a United Nations vote.

But what about the competitive impact of the withdrawals? How will this Olympics be affected as a sporting event?

Some sports, such as team handball, weight lifting, and canoeing, will be virtually dominated in the Olympics by the Eastern Bloc nations that will be in Moscow at full strength.

Others, such as yachting, equestrian, and field hockey, will be reduced to absurdity if the Western powers that historically prevail in them do not take part. (Imagine peppermint stick winning the ice cream contest in the category, "Goes best on hot apple pie.")

In the bulk of the 22 Olympic sports, there would be spirited and meaningful competition despite the boycott, but not the all-star fields and undercurrent of East-West rivalry that gives the Olympics so much of its universal appeal and glamor.

In some sports, the competition would be equivalent to that of a good international meet, but far below Olympic standards.

A brief look, sport by sport, follows.

ARCHERY: Since 1972, when archery was added to the Olympic program, Americans have won the four gold medals, men's and women's. The 1976 champions, Darrell Pace and Luann Ryon, still are competing. Rich McKinny, Rodney Baston and Lynette Johnson are other Americans who had gold medal chances. If the U.S., Italy, Sweden and Japan all boycott, the Soviet Union could clean up -- but it would clearly be a hollow victory.

BASKETBALL: U.S. men have won the gold in nine of 10 Olympic basketball tournaments, the only exception being the Soviet Union's controversial victory over the United States at Munich in 1972.

The U.S., U.S.S.R., and Yugoslavia were clearly the Big Three. The U.S. probably would have been the favorite, even with the Soviets having the home court advantage and a new tournament format that would have made two U.S.-Soviet showdowns probable.

The Soviet Union won the first women's basketball medal in 1976, and might have repeated regardless of the opposition. That would seem a certainty now.

BOXING: The U.S., Soviet Union, and Cuba are the major powers in amateur boxing. Without the U.S. and Western European compeition, the socialist comrades can beat up each other in most of the finals.

The U.S. probably would not have done as well as in 1976, when it produced an extraordinary team effort highlighted by Sugar Ray Leonard, the Spinks brothers, and Howard Davis. However, there were potential U.S. gold metalists is several weight classes: Richard Sandoval (light flyweight), Jerome Coffee (flyweight) Jackie Beard (bantamweight) Bernard Taylor (featherweight), Jeff Stoudemire (light middleweight) and Tony Tucker (light heavyweight). Heavyweight Jimmy Clark, who recently announced that he is turning pro, never beat 1972-76 gold medalist Teofilo Stevenson of Cuba, but was closer in a recent bout.

CANOEING AND KAYAKING: Neither the U.S. nor its allies is much of a factor here. The Soviets, East Germans and Romanians figured to paddle their way to most of the medals, and can be expected to do so without much effect from a boycott.

CYCLING: U.S. cyclists are improving rapidly, but did not figure to make much of an impact this year. Competitive cycling is still primarily a European sport -- East and West. The Soviet Bloc would have won its share of medals anyway, but if Western Europe boycotts, it won't have much trouble pedaling to a sweep.

EQUESTRIAN: Of the Eastern Bloc nations, only the Soviet Union is much of a factor in equestrian sports, particularly dressage and the three-day event. Without the U.S., Britain, France, Ireland, West Germany, Switzerland and other Western pillars of horsemanship, the Soviets could pick up a bunch of equestrian medals that mean nothing.

FENCING: The Soviet Union is very strong in all weapons, Poland and other Eastern European countries could be good foils. The French, Italians and West Germans, among other Western nations, will be sorely missed if they don't choose to cross swords.

FIELD HOCKEY: New Zealand and Australia finished 1-2 in 1976. India, Pakistan, Holland and Great Britain traditionally also have been strong. The U.s. men and did not qualify. The Soviets did, bud did not figure to challenge the serious field hockey powers.

Women's field hockey was added to the program at Moscow for the first time. The U.S. women qualified last summer, in an upset, and were seeded third.

SOCCER: The Olympic soccer tournament is usually dominated by Eastern European nations that call their professionals amateurs. Western countries call theirs pros pros, and they therefore are ineligible. The boycott will have little effect on the outcome.

GYMNASTICS: American Kurt Thomas, second in the all-around in last year's World Championships at Fort Worth, was the U.S. Olympian "who most had the makings of the next Bruce Jenner," according to NBS-TV's knowledgeable Olympics watcher, Peter Diamond. Even though he probably could not have defeated Soviet Alexander Detiatin for the individual all-around gold, Thomas might have repeated his World Championship victories in two apparatuses, and along with fellow "golden boy" Bart Conner, given the U.S. a men's team medal.

The U.S. edged East Germany for the team bronze in the World Championships, behind the Soviet Union and Japan, and a similar battle was in prospect again.

Without the always strong Japanese and the Americans, the Soviets and East Germans should dominate the men's gymnastics. The performance of the Chinese -- an emerging power in men's and women's competition -- would have been one of the fascinating aspects of the Olympics.

Even the boycott should not dim the interest in the women's gymnastics team competition between the Soviets and Romanians. The Soviet women had ruled the world championships since 1966 until last December, when a young Romanian team -- even with 1976 Olympic darling Nadia Comaneci out with injuries -- defeated them. The rematch, especially in the U.S.S.R., should be scintillating.

MODERN PENTATHLON: The Soviet Union has been strongest over the years in this five-day test combination horsemanship (1,000 meters over 20 obstacles), fencing (epee, one touch), pistol shooting, swimming (300 meters), and running (4,000 meters, cross-country). Poland has the individual world champion. Hungary and other East European countries are very strong, and an Englishman won the Spartakiade.

But in Budapest last summer, Americans Bob Neiman, John Fitzgerald and Mike Burley won the World Championship, an exceptional achievement. Their challenge on the Soviets home ground would have been a major story of this Olympics.

ROWING: East Germany and the Soviet Union are men's superpowers, the Bulgarians have great oarswomen, but the Americans and West Germans could have been expected to pull their share of medals. Norway and Britain also would be factors if they show up.

SHOOTING: Americans are traditionally strongest in rifle events. The Soviet Union and East Germany have very strong teams, but Western countries, including West Germany, will be severely missed.

SWIMMING: The U.S. probably would have been the strongest team in both men's and women's swimming, through the Soviets would have challenged the men, and the East Germans the women. Sweden, Canada and Australia would have provided depth in the competition.

The U.S. men had impressive hold-overs from the 1976 team that won every gold medal but one: freestyler Rowdy Gaines, Mike Bruner in the distance freestyle and butterfly, backstrokers Peter Rocca and Bob Jackson, frestyler Brian Goodell. Jesse Vassallo was the outstanding swimmer of the 1978 World Championships. But the Russians are swimming, the Russians are swimming -- and swiftly.

U.S. women swimmers were harpooned and sunk by the muscular East Germans at Montreal in 1976, but made an impressive turnaround in the '78 World Championships at Berlin. Led by Tracy Caulkins (breaststoke, individual medley) and Cynthia Woodhead (freestyle), they expected to avenge the 1972 embarrassment in Moscow.

DIVING: The Soviet Union and Eat Germany still can have a good meet, but the U.S. might have the strongest divers of all, and the Italians, Chinese, Australians and Canadians would have made it more interesting.

Californian Greg Louganis, who was second in platform diving to the great Italian, Klaus Dibiasi, (1968-72-76 gold medalist) in Montreal, might have won gold in both platform and springboard diving.

WATER POLO: Eastern Europe -- Hungary, U.S.S.R., Yugoslavia, Romania -- is very strong, but there are good Western teams as well. The U.S., with outstanding goalie Steve (Harpo) Harman, was ranked No. 2 after losing to Hungary in the closing seconds of last year's World Championship final in Yugoslavia.

TRACK AND FIELD: The U.s., East Germany, and the Soviet Union are clearly the superpowers, with West Germany probably fourth, but many nations have outstanding individual atheletes. Great Britain will be strong if it competes with world record holder Sebastian Coe (800 and 1,500 meters) and his great rival, Steve Ovett. Kenya's Henry Rono, who set four world records in 1978, seems to be back in top shape.

The U.S. had potential gold medalists in hurlers Renaldo Nehemiah (110 meters) and Edwin Moses (400 meters), long jumper Larry Myricks, decathelete Bob Coffman, all relay teams, marathoner Bill Rodgers, sprinter Evelyn Ashford and Mary Decker in the 1,500 meters, among others.

VOLLEYBALL: The Japanese are not the power they once were in men's volleyball, but a tournament without them is like a transistor radio without batteries. The Soviets, Poles, North Koreans (if they come) and Cubans will make it interesting.

The Cuban women are world champions, but the U.S. women's team had been together for two years in an effort to gain the first U.S. volleyball medal.

WEIGHT LIFTING: The Soviet Union, East Germany, Bulgaria, Hungary and Cuba should dominate, as they would have anyway.

WRESTLING: The Soviet Union is dominant, East Germany and other East European countries are strong, but the U.S. won six medals in wrestling at each of the last two Olympics and may well have done so again.U.S. wrestlers are experienced in Eastern Europe, and 380-pound Jimmy Jackson was the clear favorite for heavyweight gold, having beaten Soviet champ Soslan Andiev three times.

YACHTING: The Scandinavians, Americans, Australians, and British are the powers here.