Burnt-out cars or a sailing ship in a giant glass bottle? Meerkats or wind-powered neon signs? Don’t wait for the Turner Prize or Frieze: there will be no better place this year to see a panorama of contemporary British art than at the National Gallery’s exhibition of the models shortlisted for Trafalgar Square’s next “Fourth Plinth” installation in 2009.

A sextet of serious names in serious competition forms a dazzling line-up: leading sculptors Antony Gormley and Anish Kapoor, both in their 50s and at the peak of their careers; mixed-media political artists Jeremy Deller and Yinka Shonibare, a decade younger, who faced each other as Turner Prize nominees a few years ago; populist outsiders Tracey Emin and Bob and Roberta Smith, both born in 1963. Their proposals vary in brains, wit, seriousness of purpose and formal beauty, but each brings a lively engagement with Trafalgar Square – and thus with history and ideas of Britishness – as well as a lightness of touch to make this show provocative, fun and a compelling showcase of current trends.

Who should win? The rivalry between prominent figures has upped the game, and the stakes are high not only because this is London’s most prestigious commission, but also because these are artists who are growing up, or growing old, in public. The three former Turner Prize winners, Gormley, Kapoor and Deller, have striking, intellectual proposals that build on their well-known themes but also mark new thinking, while Shonibare, Emin and Smith offer more decorative, accessible work. Emin’s and Smith’s proposals are too flimsy to be contenders – the others have a grandeur of design and conceptual originality that could hold their own in Trafalgar Square and in different ways would transform its space and context.

Emin presents the smallest model, Smith the largest: both have been cowed by the scale of the project. Emin’s “Something for the Future” is a sculpture of four diminutive meerkats, mammals who live in an egalitarian order in the Kalahari desert. Representing Britain at last year’s Venice Biennale, Emin produced modestly sized paintings and drawings on domestic subjects that rose, in elegance and poignancy, above most loudly vacuous offerings at neighbouring pavilions, and claimed a specifically female perspective in art. She is the only woman on this shortlist (Bob and Roberta Smith is a pseudonym for Patrick Brill) and the temptation must have been to try the trick again, but the exposed Fourth Plinth has nothing in common with the cosy British pavilion, and her meerkats, whom she calls “a symbol of unity and safety”, are embarrassingly slight.

Smith’s wind- and sun-powered illuminated peace sign, constructed in collaboration with renewable energy specialists, is hardly less banal. The neon words “Faîtes l’Art, pas la Guerre” are presumably an ironic nod to Trafalgar Square’s status as a memorial to Britain’s military achievements over France, but this ladder of flashing letters looks like a gimmick in a department store window. As a challenge to the might of history and its monuments, it is conceptually and visually feeble.

Good public art, especially when it is political – four of the six proposals here deliver direct messages about peace and inclusiveness – is bold and clear-cut. Yet for a visually mature 21st-century audience it must also be sophisticated, astonishing. Shonibare’s “Nelson’s Ship in a Bottle”, a replica of Nelson’s HMS Victory enclosed in a huge glass bottle, has the surprise-and-rightness of a well-judged present: it made me laugh out loud for its charm and humour.

Instantly recognisable as Shonibare’s are the sails, produced in his trademark richly coloured batik designs bought in Brixton market, a symbol of African identity and independence. Bright and optimistic, almost too easily attractive, “Nelson’s Ship in a Bottle” alone of all the takes on Trafalgar Square’s history here emphasises harmony rather than confrontation. Shonibare says the piece celebrates British ethnic wealth and reflects “the story of multiculturalism in London today, which began as a result of Nelson’s victory at the Battle of Trafalgar”. The work would be a popular, life-affirming choice.

Another strong political contender, Deller’s “The Spoils of War (Memorial for an Unknown Civilian)” is probably too controversial to win but it is stark, devastating in its simplicity and genuinely shocking. “The presentation of the spoils of war to a curious public dates back at least to the Roman empire,” says Deller. “My idea for the fourth plinth performs a similar role. It is not an artwork, but the remains of a vehicle that has been destroyed in an attack on civilians in Iraq.” The charred, smashed red car, with its melancholy allusion to Warhol an art-historical undercurrent, would bring contemporary world politics into Trafalgar Square as no other Fourth Plinth occupant has: a brilliantly ironic proposal, and one that raises key questions about the role of art in public life.

As confident as Deller in dispensing with aesthetic concerns here, Gormley is more subtly political. “One and Other”, a suggestion that the empty plinth be occupied 24 hours a day by volunteers standing on it for an hour at a time, brings to its apotheosis his lifelong concern with representation, in terms of both figurative art and the democratic process. It also confirms, as his Hayward show Blind Light did last year, that Gormley has always been a hard-hitting conceptualist – sometimes disguised in beguiling figurative clothing – and that the religious/puritanical strain in his work is getting ever stronger.

His proposal that “through elevation on to the plinth and removal from the common ground, the subjective living body becomes both representation and representative, encouraging consideration of diversity, vulnerability and the individual” is a smart riposte to Trafalgar Square’s military statues and cleverly uses his audience as found objects. Yet this vision of a bleak, lonely plinth – like the searing emptiness of the “Blind Light” installation – and Gormley’s mix of populism with a lofty, nihilistic refusal to make art, maddened me, and I found myself hoping that “One and Other” does not play out its end-game on Trafalgar Square.

For three decades, Gormley and Kapoor have challenged each other, as well as the boundaries of sculpture, and the most compelling dialogue here is the stand-off between them. Kapoor’s “Sky Plinth” is an assemblage of five concave mirrors that cantilever off the plinth, treating all its faces as supports. In the mirrors’ reflections, the clouds would descend and “Sky Plinth” would display the changing sky-scape “as a monument on Trafalgar Square”. With the plinth an object dematerialised by the mirrors as they turn the world upside down, the effect would be a continuation of the unique metaphysical language – contrasts between light and dark, interior and exterior, heaven and earth – that shapes Kapoor’s work.

Kapoor is a sculptor of sensuous, curvaceous shapes, rich colour – two of “Sky Plinth’s” mirrors are a translucent purple and red – and an enigmatic, abstract vocabulary. Gormley is an ascetic, representational artist who on the surface speaks more plainly, with a political agenda. Yet their juxtaposition emphasises a twinning too, in the spiritual resonance and absolutist drive that make their proposals the most densely fascinating of an impressive shortlist.

My vote goes to Kapoor: for the sheer beauty, thought and ambition – to bring the sky to the earth – of “Sky Plinth”, and because, alone of all the artists here and in a climate where concept is increasingly privileged over individual vision, he asserts art’s formal qualities and its capacity, as eternal as the sky, for transcendence.

Fourth Plinth, National Gallery, London WC2, to March 30. Tel: +44 (0)20-7747 2885, www.nationalgallery.org.uk