AMADEUS -- At the National through December 6.

Here, at last, is the justification for hoarding theatrical superlatives -- for having resisted the temptation to characterize the ordinary run of amusing or interesting plays as "brilliant" or "dazzling."

If "incredibly witty" is used for such pleasant fluff as "Lunch Hour," or "magnificently operatic" on the good craftsmanship of "Sweeney Todd," what is left for "Amadeus," which is, as it happens, incredibly witty and magnificently operatic, not to mention brilliant and dazzling? It is also -- to complete emptying the treasure room of the adjectival warehouse -- poetic, comic, deep, moving and acted to perfection.

It's good, too.

Peter Shaffer's play, a London hit having its pre-Broadway engagement at the National Theater through December 6, is about Mozart. But the reason Mozart's middle name is used for the title is that, rather than being ordinarily biographical, the play is about the miracle of divine talent's occurring in an otherwise slight and silly human being. Amadeus: God's darling.

This Mozart got his looks and brains right from the top of the Salzburg chocolate box. Tim Curry is frightfully funny as a bouncy, conceited, childish twit, "Leopold Mozart's swanky son," so devoid of intellectual or artistic qualities that his ability to create instant perfect music can only mean that God is using him as a transmitter.

The tragedy, therefore, is not Mozart's, in spite of his early impoverished death. A more original tragic hero -- played by Ian McKellen, in as fine an acting performance as one can ever hope to see -- is the historical character of Antonio Salieri, whose music brought him all the honor, money, and royal and popular acclaim that eluded Mozart in his adult life. But Salieri alone understands that Mozart's existence makes his a mockery.It's like the old joke of thinking your accomplishments impressive and then realizing that at your age, Mozart had been dead for years. "Now for the first time, I know my emptiness, as Adam knew his nakedness," Salieri informs God -- God, not Mozart, being the object of his outrage. He had made his bargain with God -- a God "staring at the world with dealer's eyes" -- and was cheated.

It's hard to avoid sharing the resentment of Salieri, now knowing himself to be "the patron saint of mediocrity," who pledged virtue, devotion and work in return for what was given instead to a misbehaving squirt. dWe see the banalities of Mozart's life become immortal. From cranky, adolescent rebellion "rose the ghost of Don Giovanni"; from his nauseating baby talk with his wife, the Papageno-Papagena duet. "From the ordinary, he created legends, and from legends I created the ordinary," Salieri acknowledges bitterly.

It is Salieri's special curse that he has the musical descernment to know, as his contemporaries do not, the extent of Mozart's genius. In a panoply of powerful acting, McKellen is not least impressive when he silently hears Mozart's new works with a polite but unmoved audience. "Through my tears, I saw the Emperor yawn," he says after the premiere of "The Marriage of Figaro." The irony of his own life is that he is "called distinguished by people incapable of distinguishing"; his punishment that "I must survive to see myself become extinct."

The play is written just that beautifully throughout. Shaffer, a music critic before he wrote "Five Finger Exercise" and Equus," has humorously given "Amadeus" the structure of an opera, opening it with a two-person spoken chorus and including a hokey, messenger-in-disguise climax. But the story of Salieri's anguish, although comically told, is startlingly fresh and powerful for thought-provoking literary drama, let alone for opera.

The production is done by people with strong Shakespearean backgrounds, and it shows. Sir Peter Hall, the director, is also a director of opera, and has managed to suggest that form without making it overshadow the importance of the words.In addition to the peerless McKellen and the delightful Tim Curry -- whose credits also include "The Rocky Horror Show" -- there are deft and devastating performances by Jane Seymour as Constanze Mozart, Nicholas Kepros as the Emperor, Patrick Hines as Count Orsini-Rosenberg, and the whole supporting cast. Even the non-speaking walk-on parts are memorable, such as the way Linda Robbins as Salieri's wife has of placidly listening to music.

God does not actually appear, although McKellen may convince you that he does. But the nya-nya voice with which he is quoted as saying, "I don't need you, Salieri -- I have Mozart!" indicate a performance up to the standards of the production.