Flamboyant home for modern art; West Germany's state gallery opens controversial wing.

''ES ist Metro-Goldwyn-Mayer?'' This tart question - ''Is it MGM?'' - was posed by Der Spiegel, the prestigious German weekly, last month upon the unveiling of Germany's newest, most flamboyant, and very controversial museum, the Neue Staatsgalerie.

Bright orange revolving doors lead into a foyer with a translucent green, raised-dot rubber floor that visually screams but audibly hushes sound. The reddish-beige sandstone exterior is slashed with diagonal walkways and punctuated with bright magenta balustrades. A wavy-glass facade worthy of a 21 st-century greenhouse is held tightly by fingers of green-painted spring steel. Inside: high ceilings, spacious carpeted rooms with domed atriums for natural light. Outside: constructivist canopies, a circular, classical-statue terrace with angular windows and an ascending, sickle-shaped walkway.

The new $35 million wing of the state gallery that opened here March 9, which was a decade in the making, houses West Germany's most prestigious modern-art collection - including, but not limited to, German artists with such esteemed reputations as Edward Kienholz, Joseph Beuys, and Oskar Schlemmer.

The design, by British architect James Stirling, won an international competition in 1977 and is an addition to the neoclassical gallery that was built in 1837.

The museum is joined to its 147-year-old predecessor and is smack across downtown's Konrad-Adenauer-Strasse from the old and aging State Opera and State Museum buildings. Its iconoclastic look was ostensibly chosen by museum organizers because it represents a way forward for the city's architecture. The design is being labeled ''contextualism'' - responding to the history of its neighboring buildings, while at the same time leaving behind the drabness that has blighted many European cities since World War II. Universal and functional approaches are ''out,'' the rationale goes; particular and romantic regional approaches are ''in.''

The handful of reviews from German newspapers since the unveiling have been largely favorable. ''Elegance and humor,'' said Munich's Abendzeitung. ''Collage is the answer to loss of style,'' says the Badische Zeitung. And the famous American architect Philip Johnson has called architect Stirling ''a master of Weltarchitektur (world archictecture).''

But architecture critics from as far away as London say that the building ''determinedly ignores nature'' and is ''manifestly concerned with the private theories of the architect.'' The English Journal Architecture Review called Stirling's building ''crazy.''

And many of the 36,000 visitors that paraded on its three floors and numerous terraces in the first three days were wary. ''It sticks out like a sore thumb, '' said an elderly man who pointed to the ''verruckt'' (crazy) combination of stone, glass, and color. One woman with a child wanted to know why the building for artwork should draw so much attention to itself rather than the art treasures inside. Unlike the grown-ups, however, children seemed to like the building more than the art.

Architect Stirling himself has written that he hopes the Staatsgalerie is monumental but also ''informal and populist - hence the anti-monumentalism of the meandering footways and the voided centre and much else including the colouring.'' He welcomes ''the passing of the revolutionary phase of the Modern Movement'' (the Bauhaus and International styles) as ''repetitive, simplistic, and confining.'' And he says the building looks as a museum should. ''In addition to representational and abstract, the Staatsgalerie I hope supports the monumental and informal, the traditional and high-tech.''

Fitting snugly against a hill that juts up sharply behind - and squeezed between a row of older homes at one end and the older gallery at the other - the building is less noticeable than it might be. It was also built on a similar scale to that of the surrounding buildings, and with similar materials. A flashy, neon-lit restaurant and chamber theater are also integrated into one end of the museum's structure.

But if the architecture is so far getting all the attention, it will not be long, says one official, before people will settle down to appreciate what the director, Dr. Peter Beye, calls the best collection of modern art in West Germany. Wandering the 15 spacious display rooms (about 12,000 square feet of hanging space), one finds nary a room without a major standout: In sculpture, the six figurines of Oskar Schlemmer's ''Figurinen zum Triadischen Ballett,'' Joseph Beuys's bizarre ''Dernier espace avec introspection,'' and Edward Kienholz's ''The Birthday'' are the major attractions. Six wooden figurines by Picasso standing in gravel are titled ''Die Badenden'' (''The Bathers''). Hans Arp, Alberto Giacometti, Sol Lewitt, and Duane Hanson are all generously represented. Schlemmer has more paintings than anyone else. But Paul Klee, Jackson Pollock, Kurt Schwitters, Egon Schiele, Kathe Kollwitz, and Wassily Kandinsky are all represented as well. In the off-white display rooms themselves , there is nothing to compete visually with the artwork, which is labeled only in German.

It is an architectural plan, both inside and out, says the minister-presi-dent of Baden-Wurttemberg, Lothar Spath, that wil make the Staatsgalerie ''one of the greatest buildings of this century.''