THEY PICKED the most beautiful stamps in the world in Washington last week. The people who did the voting are the people who should know: They print most of the world's stamps.

The occasion was the Government Postage Stamp Printers' Conference and as Carl D'Alessandro, assistant director for operations at the U.S. Bureau of Engraving and Printing, put it: "We know an outstanding stamp when we see one."

There was no overall winner, but the U.S. printers, who organized the 21-nation conference, walked off with two of the six prizes.

The honors took on special significance because, as bureau director Robert J. Leuver noted, it was the first time the printers had held a competition among their peers.

The winning U.S. stamps were:

The 1986 set of four stamps celebrating stamp collecting, picked by the printers as the best multiple stamp issue from sets of stamps entered by seven countries. The same set also was picked by American collectors as the best U.S. issue of 1986 in the annual Linn's Stamp News poll.

The 1985 commemorative honoring Frederick Auguste Bartholdi, sculptor of the Statue of Liberty. It won honors as the "most innovative or unique" because it was created in the Bureau of Engraving's "electronic imaging design center." The computers there allow printers to adjust the colors and elements in a stamp electronically so they know how it will look before making test runs.

Sweden, renowned for its finely engraved stamps, won the intaglio, or engraved, competition for a multicolor of a tall ship at anchor. It defeated six others, including the U.S. 1986 Statue of Liberty.

Italy won the gravure category for its scene of Capri on a 1986 stamp. Six countries had entered this category, including the United States, which submitted its 1985 Christmas Madonna stamp.

Portugal captured the offset competition with one of the stamps from a sheet featuring brightly colored street kiosks. Korea, Japan and Hungary also entered this one.

France won the category of best stamp produced with a combination of printing techniques for its stunning rendering of a stained-glass window from the Cathedral of Strasbourg, reproduced by an offset and intaglio process. The United States submitted its 1985 commemorative celebrating the Ameripex stamp show. Five other countries also entered the category.

Each of the 18 countries that joined the conference prior to the meeting was eligible to nominate stamps for the competition. The countries each were given one vote in each category and were barred from voting for their own stamps.

Only nations that print their own stamps may join the conference. That bars such countries as Canada and the United Kingdom, which are popular with U.S. collectors but use commercial printers. It also rules out many small nations that churn out flashy stamps designed primarily for sale to collectors -- stamps which one guest at the awards ceremony called "wallpaper."

Leuver organized the stamp conference last year as a way to bring printers together to discuss production problems and new technology.

The group will meet again in 1989 in France. While computers rapidly are changing the technology of printing, the highlight of the conference is almost certain to be the contest for the best-looking stamps.

The Washington International Stamp Exposition takes over the Twin Bridges Marriott off Shirley Highway in Arlington this weekend. The show runs 10 to 6:30 Saturday and 10 to 5 Sunday. Admission is $2; retirees and children under 16 get in free.

Bill McAllister is a member of The Post's national staff.