Let's come right out with it: on the basis of his installation of one old and seven new rolled steel sculptures at the Guggenheim Museum in Bilbao, we can call Richard Serra not only the best sculptor alive, but the only great one at work anywhere in the early 21st century.
In its short history, American art - to be somewhat categorical about it - has produced three truly outstanding sculptors. (There are other contenders, of course, like early Calder, Donald Judd or Robert Smithson, but these strike me as the indispensable three in terms of beauty and cultural impact.) First, there was Augustus Saint-Gaudens. The son - despite his French name - of Irish immigrants, he gave marble and bronze form to the serious ambitions, both monumental and decorative, of America's late-19th century classical revival, as John Singer Sargent didin painting and Stanford White did in architecture. Not even Rodin's Burghers of Calais is a nobler commemoration than Saint-Gaudens' Shaw monument in Boston, honouring a regiment of black volunteers who fought for the North in the civil war.
The second was David Smith (1906-65), who, while he did not exactly create the 20th-century idiom of welded steel sculpture (as distinct from modelled clay, carved wood or cast bronze), did more than anyone else in the mid-20th century to expand it to an industrial scale and endow it with a formal range and expressive strength it never had before. It was not Smith's fault that his legacy of open-constructed steel eventually turned into a sort of corporate cliche, logos for a thousand lobbies and malls. Shit happens, as they say in America.
And the third is Richard Serra, a steelworker, too, but not a constructor like Smith. Now 65, Serra has embarked on a magnificent, productive maturity. Put in the simplest terms - ones that Serra might find too simple, but never mind - his achievement has been to give fabricated steel the power and density, the emotional address to the human body, the sense of empathy and urgency and liberation, that once belonged only to bronze and stone, but now no longer does. He has achieved a very deep synthesis, and it may not matter whether others follow him. Once you are in the enormous Guggenheim gallery which these sculptures fill, once you are absorbed in their space and pacing out their convolutions, you feel suddenly free - far from the dead zone of mass-media quotation, released from all that vulgar, tedious postmodernist litter and twitter, from the creepy posturings, tired bad-boy claptrap and squalid sanctimony that characterise PoMo and BritArt. It is quite a good feeling - rather like the old days, one's inner fogey is tempted to say. The work is as new as new could be, but when you are experiencing it you may also think of an 18th-century definition of the spirit of classical sculpture: "A noble inwardness," wrote Johann Winckelmann, "a calm grandeur." Eine edle Einfalt, eine stille Grösse . Without the white gods, of course.
"Do not speak to me about small projects," said Gianlorenzo Bernini to Louis XIV, when the sun king brought him in state to Paris to redesign the Louvre. This could have been Serra's motto at Bilbao. The design of the museum by Frank Gehry was a gamble for the industrial capital of the Basque country. Its high cost and eccentric, ultra-mannerist idiom, its asymmetric spaces and glittering titanium skin, did a lot to draw world attention to Bilbao's project of reviving itself. But would the city be left with an enormous, world-famous box with less than first-rate contents - a pyrrhic victory of packaging over substance? This seemed ominously possible, when one learned that the Guggenheim had been commissioning whole rooms as long-term or even permanent installations for Bilbao from some quite minor artists, such as Francesco Clemente and Jenny Holzer.
Serra's installation is of quite another order. The gallery it occupies is the biggest in the museum - a vast room, some 430 ft long by 80 ft wide. Paintings hung in it before, and they usually looked diminished by Gehry's architecture - sometimes to the point of silliness or near-invisibility. But Serra's work dominates Gehry's space like a rhinoceros in a parlour. (There's said to be considerable animosity between the two men; if that's so, one certainly knows, in this case, who the winner is.)
Serra's material is steel sheets, about two inches thick and up to 50ft long and 14 ft high. These are curved in a rolling-mill along both the horizontal and vertical axes, as though they were mere sheets of tin. Of course, they are no such thing: they are so immensely heavy that only one rolling mill in the world (at Siegen, in Germany) could handle them. These sheets are joined by spot-welds to form curling walls. This creates a passageway, through which you move. The walls lean and straighten; they reverse their curvature, bulging and then receding. Nothing supports them but their own weight, bearing hugely and mutely on the floor.
The sheer courage of these pieces is breathtaking. The space inside, the gap between the walls, narrows, widens, breathes in and out (if you can speak of massive iron "breathing", which in Serra's work you can) and eventually rewards you with an inner chamber, from which you have to follow the same route out. At all points these constructions are open to the upper air, the gallery roof (and hence the architecture of the gallery) or the sky. But you can see out of them only by looking up, which doesn't really help you locate yourself. You would think it would be claustrophobic, terrifying, to be in the narrow curving slot between these giant planes, to be unable to see what lies ahead. Indeed, the fear of being crushed like a bug on an anvil has always been present in responses to Serra's work, a bass vibrato at the edge of consciousness. But you have to trust him, or lose the work in its entirety. There is just no way of experiencing these pieces by looking from the outside, or in photos, or on video: the initial view of them from the balcony above the Arcelor gallery is impressively dramatic, yet it's the merest pipe-opener to what unfolds close up and at floor level.
It is important to know that these "torqued ellipses", spirals and toruses, these gigantic exercises in topology, are not intended to be mazes. There are no choices about which way to go. Each has only one way through it, an end, and the same path back; each emphasises the ancient Greek philosopher's Zen-like adage: hodos ano kato mia kai hote , "the way up and the way down are one and the same". A maze would be fussy; it would interfere with the stupendous directness and logic of Serra's spatial language.
There are a few - a very few - other living visual artists who are able to create such structures, in which a marvellous complexity unfolds almost of its own inexorable will and nature from apparently simple premises which, once they are granted and enunciated, generate the form. The most readily apparent of them is the great architect Norman Foster, who has a similar trust in the working-out of process. This process does not guarantee sublimity; that is never guaranteed. But it is perhaps the main path to the sublime, now that the gods are dead and so much of nature herself is dying, and that is the direction in which Serra's work is set. It is also the source of the continuous surprise afforded by his sculptures, as you move through them. How can things this big and so apparently simple be so unpredictable? Just look.
The curious fact is that when you do look, what you see is not only sculpture but a kind of painting as well. Of course, all sculpture has colour, or colours. But Serra's work, in all the gloomy or lambent richness of its weathered steel - now as red as one of John Ford's sunset buttes in Monument Valley, now as black as the hull of a stricken tanker - is singularly enriched by colour. The colour, one must add, is not applied. There is no painting or artificial patination. Serra even claims he doesn't pick his plates for their colour, which may be true but is a bit hard to credit. But when you are between the steel walls you gradually realise that this living, streaked, mottled, accidental darkness conveys the sense of inborn, embedded emotional tension that Mark Rothko tried, but ultimately failed, to produce in his secular chapel in Houston; that it is the "real presence" of the metaphors of nature which, in Clyfford Still's awkward hands, turned into parody. And then you see how woefully insufficient it is to call Serra a "minimalist". None of the labels applies to this work. Labels are a nuisance, a distraction. But if you wanted to use one, you could just as easily call Richard Serra the last abstract expressionist.
· At the Guggenheim, Bilbao. Details: 00 34 94 435 9080.