ASTRONG argument can be made that the Napa Valley winery of Joseph Phelps is consistently pro- ducing more fine wine than any other winery in this famous and beautiful viticultural region north of San Francisco.

Virtually every wine enthusiast has no doubt heard of the Joseph Phelps Winery. However, given the impeccable quality of its wines, and the current track record of top-notch wine successes, which extends back through the 1975 vintage, it remains somewhat surprising that the Joseph Phelps Winery has not received even more publicity.

Part of the answer is certainly because the winery's founder and owner, Joseph Phelps, seems to prefer to let his wine do the talking rather than himself.

Phelps' rise to the top of the crowded Napa Valley wine scene in 1984, with what must be the greatest lineup of fine wines in the winery's 10-year history about to be released, is hardly surprising. But in fact, it was done without famed winemaker Walter Schug, whose talents for producing fine wines first made people take notice of the Phelps label. Schug parted company amicably with the Joseph Phelps' Winery several years ago to produce wines under his own label, and his former assistant, Craig Williams, has improved upon even the illustrious efforts of his highly respected predecessor.

Phelps, a Colorado builder who had constructed the impressive Souverain Winery facilities in the early '70s, built his own 65,000-case winery in 1974. The early raves from critics were usually for the winery's white wines, particularly the late harvest rieslings, which the German-born and trained Schug seemed a natural to produce.

However, in the 1975 vintage, a year not considered particularly outstanding for the red wines in Napa Valley, Phelps produced four superlative red wines. Released to spectacular reviews from wine critics throughout the country were a regular Cabernet Sauvignon (called by many critics the best in its category and price range), a special vineyard-designated cabernet (from purchased grapes of the Eisele Vineyard) that is extremely complex and concentrated, and a gorgeously textured, profound zinfandel from grapes purchased in Alexander Valley, Sonoma County.

Immediately, the Joseph Phelps Winery came under close scrutiny from wine enthusiasts, not only for its excellent white wine, but also for a new breed of serious, very rich, concentrated and tannic red wines.

The successful red wines of the early years of Phelps' Winery were not flukes. The winery has continued to turn out exquisite cabernets under the Insignia label and the Eisele Vineyard label, both of which have set the standard for the best that Napa Valley can produce.

Even the regular cabernet, though there has never been one to quite match the sensational 1975, remains one of the better regular bottlings by any winery in California. The zinfandels from the Alexander Valley have been consistently among the best made in California, with a complexity and balance rarely encountered.

Pinot noir, the fickle grape that Schug worked so hard with to perfect, has been dropped from the Phelps' portfolio. However, Phelps' obsession with producing real syrah wine from this famous grape that renders France's fabled Rho ne wines of Hermitage, Co te Ro tie and Cornas, has reached surprising new heights in bouquet and taste. In California, many wineries make a red wine from the petite syrah grape, but Phelps is one of the few wineries to own 20 acres of the true syrah grape. His newest wines from this grape will no doubt provide a major breakthrough for the possibilities of this varietal in California.

The stunning success of the red wines still does not outshine the winery's touch with white wines. The gewurztraminer has always been one of the best made in California. The reislings, which set a new standard for elegance and Germanic likeness under Schug, continue to be exceptional, not only the lean, steely, early harvest rieslings, which resemble a fine German kabinett or spatlese, but also the honeyed, decadently rich and sweet late harvest rieslings, which have enough sugar and complexity to rival the best of Germany's famed trockenbeerenauslese wines.

However, the new star in the white wine framework of the Joseph Phelps' Winery is the sauvignon blanc. Until 1983, this wine was made in a moderately grassy, Loire Valley style. But beginning in 1983, Williams moved to a crisper, dryer, less grassy style, which if the 1983 is any indication, will be a huge critical and commercial success.

Joseph Phelps, who seems remarkably modest, yet quietly confident and proud of his wines, is in my opinion at the very top of his prestigious class in the competitive Napa Vally. From his inexpensive white and red table wines called vin blanc and vin rouge, to his expensive and profoundly rich and interesting cabernet sauvignons, zinfadels, syrahs, and late harvest rieslings, there has been one top-flight wine after another for almost a decade.

His new releases this fall take the already exceptional quality one step higher.