AS PROGRAM annotators never tire of reminding us, the happiest moment of Leopold Mozart's life was a Saturday evening in March or April of 1785, when Joseph Haydn said to him in Vienna: "Before God and as an honest man, I tell you that your son is the greatest composer known to me either in person or my name . . ." What moved Haydn to make that famous declaration on that particular evening was the first hearing of the last three of the six string quartets Wolfgang had dedicated to him and published as "Opus X." The performers, as in so many of those marvelous after-hours sessions, were Haydn himself, as first violin, Karl Ditters von Dittersdorf as second, Mozart playing viola, and Jan Krtitel Vanhal as cellist.

The six quartets dedicated to Haydn, composed between December 1782 and January 1785 (K. 387 in G; K. 421 in D minor; K. 428 in E-flat; K. 458, the "Hunt," in B-flat; K. 464 in A, and K. 465, the "Dissonant," in C), have been over the years the most successful and beloved of Mozart's works in this form. Whether they are, collectively or individually, as great as the four later quartets is rather beside the point: all six are masterworks, and any one of them would have made a name for its composer if he had never written anything else. They have always been well represented on records, and at present there are, in addition to recordings of various individual works from this cycle, no fewer than nine "integral" sets of the six competing for attention.

In the lowest price category, there is a mono set by the Budapest Quartet, originally issued on Columbia in 1953 and available now as Odyssey Y3-31242 (three discs), and there is an early-stereo set by the Hungarian Quartet on Vox (SVBX-589). While neither is less than serviceable, neither, in the face of the really formidable competition, has much more than economy to recommend it, and the more brightly recorded performances by the Bulgarian Quartet available for the Musical Heritage Society (OR-175, 176, 177), which cost very little more, strike me as a more satisfying alternative in the "bargain" class.

There are no real bargains in this material, though: it seems you simply do pay more to get more. You don't get a lot more from the Guarneri Quartet on RCA, but the step-up is more noticeable in Telefunken's recordings of the Alban Berg Quartet of Vienna, and at the $8.98-per-disc level we encounter three truly distinguished presentations - those by the Amadeus Quartet on Deutsche Grammophon, the Quartetto Italiano on Philips, and the Melos Quartet of Stuttgart, also on DG.

Both the Amadeus and the Quartetto Italiano have been performing for more than 30 years without any change in personnel, and with these works always prominent in their repertoires. Their playing is elegance incarnate, with the Amadeus a little crisper (also stingy with repeats), the Italiano more expansive. The Melos Quartet seems to combine the virtues of both of its illustrious predecessor groups, and to benefit from richer, more up-to-date sound; elegance and vigor are nicely balanced, and first-movement repeats are consistently taken. So far only the first four of the six quartets have appeared in this new series (DG 2530.898 and 2530.800), but the third disc may well be out by the time these words appear in print. In the meantime, the more than durable Amadeus performances are packaged with the last four quartets in a specially priced five-disc box (DG [WORD ILLEGIBLE] ) which has earned a place of honor in the Mozart discography even though some or all of its contents may be excelled by the Melos and other more recent versions.

There is, in fact, another set, still more costly ($14 per disc, list), which does yield still deeper pleasure. The Smetana Quartet, whose performances of Mozart's Quintets in C major (K. 515) and G minor (K. 516) with Josef Suk as first violist made such an outstanding impression in Nippon Columbia's Denon digital series (OX-7089-ND), has recorded the six "Haydn" quartets on the same label with similarly outstanding results.

If I'm not mistaken, the Smefana Quartet has been playing as long or longer than the Amadeus and Italiano foursomes, also without personnel changes. In any event, the Czech performances are as idiomatic, refined and spirited as those of the other two senior groups (also without first-movement repeats), and the sound itself is an enormous asset. There is simply no other recording of these works (or of much else) that compares with Denon's in terms of spaciousness, natural balance and overall realism. The combination of this quality of sound with this quality of performance is one of those classic examples of the whole greater than the sum of its parts.

So, too, is the packaging of the six works as a unit. Denon has issued the six quartets on three separate discs: K. 387 and K. 428 on OX-7034, K. 421 and K. 458 on OX-7008, K. 464 and K. 465 on OX-7039. In Japan, but so far not in this country, the recordings are also offered in straight chronological sequence in a three-disc set (OB-7341-3ND) at a special price. Why the cumulative effect of this packaging seems to enhance one's pleasure still more is a fine psychological point, perhaps, but if the special price arrangement could be effected for U.S. release of the set (in this case, say, $35 instead of three times $14) the benefit would be more than psychological. And what a Christmas gift this set would make for someone you really want to impress - or really love.