NOTHING can prepare you for the impact of walking into Ron Mueck's show, the runaway hit of this year's festival. I have never seen audiences so completely rapt in an artist's work. They gaze, not with the sort of fixed stares dutifully accorded Monet and van Gogh, but with a genuine curiosity and a real sense of wonder.
When I visited, the interpretation room, with its displays of Mueck's maquettes and a video explaining his techniques, was packed. But there is no simple explanation. For Mueck's work reaffirms the magic of art. And it does so by making us look hard at a subject which many artists choose to eschew: the human body.
Mueck's bodies are unlike any in art that you will have seen before. Certainly you might compare them to the super-realist work of Duane Hanson or John de Andrea, but Mueck goes a stage further.
His technique of modelling full size in clay ensures that he is able to replicate at an early stage every wrinkle, stretch, pimple and goose-bump to an unprecedented degree. Once the sculpture has been cast in a resin whose liquid mixture sets with a translucency which perfectly replicates that of skin, he finishes it off with pigment and hair, inserting every strand into its designated follicle.
Mueck's creatures are, universally, ill at ease. A gawky teenage girl in a swimsuit leans with her back against a wall, uncomfortable with her ungainly proportion, betrayed by her big feet. A giant bearded man, exposed in his nakedness, sits staring straight ahead like a displaced inmate from a lunatic asylum. A huge woman sits propped up in bed, her eyes wide with a fearful anticipation echoed palpably in every inch of her being. A miniaturised pair of lovers, their diminutive scale reinforcing their vulnerability, lie awake beside each other in bed, exuding tension and a sense of deep unease which provoke us to invent a narrative.
Even the most recent work here, a newborn baby, modelled at perhaps 20 times life size, displays the disquietude with which all of us enter the world. But the most revealing piece, and the most haunting, is not one of the giants but a small, one-third scale sculpture of a naked man sitting in the prow of an oarless rowing boat. An anxious look clouding his features, he cranes his neck to see ahead. Clearly, and unusually obviously for Mueck, he is intended as a metaphor for the human condition.
The interpretive video explains how Mueck, seeing a Murillo, noticed retrospective parallels between Man in a Boat and the notion of the Virgin as a vessel.
But there is more here surely in common with the Romantics, in particular with Friedrich and, looking at Mueck's babies, it is impossible not to think of the otherworldly children of Runge and Messcherschmidt.
There is also a beautiful, essentially Romantic irony in the fact that Mueck works so hard to ensure that his creations should look so very shabby. They are avowedly realist, diametrically opposed to classical, idealised notions of the human form.
But there is also something about them which works beautifully with the grand, neo-classical proportion of these galleries. A sense of imperfect man inhabiting the perfect space of his own creation, which in turn sends out a fundamental message about the impermanence and imperfection that is all we can expect from our own all-too-brief lives.
Mueck's preoccupation with the inexhaustible possibilities of the figure is echoed across town in the exquisite oil paintings of Moyna Flannigan. Building on her recent work in which she explored the world of male cross-dressing, Flannigan offers us unsettling images of curious, doll-like women from the demimonde of fetishistic sex.
What removes her images from the realm of the merely salacious, though, is their wit. Her characters are caricatures of sexual and social stereotypes adrift in a world which appears as a tasteless hybrid between the commedia dell'arte and burlesque. While mostly they have an element of realism, on occasion Flannigan allows her imagination free rein and creates monstrously deformed creatures whose purpose is metaphorical.
It is testimony to her talent that while clearly sardonic, these hugely engaging works manage to elicit our sympathy and compel us to acknowledge the sordid reality which is their inspiration.
That very reality, in all the raw, unforgiving detail of voyeuristic sleaze, is intrinsic to the work of Albert Watson. Watson, the boy from Edinburgh who left to become one of the world's great photographers, returns in triumph to his native city. The result is an absorbing voyage through late 20th-century culture, mixing celebrity gloss with low-life thrills.
Watson has hung the show himself, deliberately out of any chronological order, so it is hard to discern his progress. But take time and you can see how it happens. How he gets from a posed 1973 photograph of Hitchcock with a plucked bird, through late Seventies reportage from Beijing, fashion shoots and glitterati portraits, to a recent 2004 sequence of a nude girl garlanded with thorns.
These are huge images, some around 5ft high, which hit you with the impact you might normally expect from a painting. Here, as with Mueck, size matters, making it impossible to ignore Watson's graphic subject matter.
There is an undercurrent of sheer sexiness flowing through Watson's work. He delights in texture - deliberately comparing Mike Tyson's back with the scarred surface of a Stone Age pestle and mortar. One of his salient motifs is the line of the nude human body which he exploits, contorts and explores to the full. Watson has a vivid sense of pure beauty, often treating a woman's nude form with the same fascination and reverence he affords a desert landscape. Among the most arresting images are two simple photographs of colourful jellyfish floating against a black background.
At the same time he revels in kitsch, focusing with unrelenting candour on he world of the motel and the pole dancing club. This exploitative environment is further invoked by his muse, Breaunna, whom he delights in photographing nude or half-naked, wearing bizarre fetish costumes, in domestic interiors.
Yet it is probably not for these exotic, luxuriously decadent images that Watson would like to be remembered. Clearly, from the space accorded them here, he is most proud of a 1998 series of photographs made in Morocco and certainly, if a little more prosaic than most, they must rank among his best work.
Principally for me though, Watson's real legacy lies in the compelling picture he offers us of his adopted home of America at the turn of the 20th century. In particular his shots of Highway 15, near Las Vegas, seem to capture his own love-hate relationship with all the contradiction and confusion of that multi-faceted society.
Harry Benson, like Watson, left Scotland to work as a photographer in the States and made his name by being in the right place at the right time. But that is where their similarity ends.
Benson's photographs do not impose on you as Watson's do. They are much smaller in size and, while Watson relies on careful composition and posing, Benson's business has always been to capture the moment. That is not to say that he cannot compose. Margaret Thatcher sits before a portrait of Wellington, Caroline Kennedy stretches out to fill the picture plane with an arc of white tulle, as she rushes towards her wedding.
But it is spontaneity which is Benson's trademark. So, while Watson may seduce you, Benson has the power to stop you in your tracks. He wants you to look at his people - at the Clintons about to kiss; at the father waiting for a plane after burying his son killed in Vietnam, as he cradles in his arms the Stars and Stripes from the coffin; at the bewildered young electoral campaign worker, standing in a pool of Bobby Kennedy's blood minutes after his assassination.
The exhibition is titled 'Being There', but it was not just a case of Benson having been there at the right time. His uncanny knack of knowing just when to freeze the expression enables us to reach down into the uncomfortable humanity of his subjects. And this in a sense is where we came in, with Mueck's awkward giants and midgets. For, while Benson may be described as a reportage photographer, what he really does is to show us the famous as mirrors of ourselves, imbuing them with an ordinariness which offers us not just a sense of what life is all about, but a tantalising glimpse what it might become.
• Ron Mueck, RSA Playfair Building, until October 1; Moyna Flannigan, doggerfisher, until September 30; Albert Watson, City Art Centre, until October 22; Harry Benson, Scottish National Portrait Gallery, until January 2007