Atop Trafalgar Square's once empty plinth at present is Thomas Schütte's Model for a Hotel 2007. Schütte's model collects the light available to it from the open public square, reflecting it through the edges of its primary coloured, horizontal glass panes (top left, above). The model looks deceptively lighter than it is (weighing in at just over eight tonnes), not least because it is translucent against the sky. In this way it speaks to the historical weight of the buildings that surround it, thereby also drawing attention to its temporary nature.

Every year, Trafalgar Square's Fourth Plinth showcases the work of a well-known artist, each facing the creative, temporal, historical and social challenges this public commission presents. Now on show at the National Gallery are the six new proposals for the Fourth Plinth, commissioned from artists Tracey Emin, Antony Gormley, Jeremy Deller, Yinka Shonibare, Bob & Roberta Smith and Anish Kapoor.

Bob & Roberta Smith's proposal is perhaps the most glamorous of the bunch - an illuminated peace sign reading "Faîtes L'Art, pas La Guerre" (Make Art, Not War). It is to be powered by the sun and the wind, and is consequently the most collaborative in its making, involving a dialogue between renewable energy specialists, structural engineers and an architect. The physical mechanics of this collaboration are all on show as part of the light sculpture itself.

Antony Gormley's piece also involves collaboration, but on a level that is judged far more perfunctory by the artist. Gormley proposes that the plinth be occupied 24 hours a day by members of the public who have volunteered to stand on it for an hour at a time. His proposal is characteristically philosophical and demanding in the relationship it seeks to have with the public. The feasibility study that accompanies his proposal reveals the mechanical finish he would like the complicated and potentially personable logistics of this piece to have. For Gormley, it seems, it is not important what the public choose to do, rather that they are doing what they are doing because he has facilitated it. One might go so far as to say that Gormley's trademark body cast has transmuted on this occasion into the plinth itself. He carries the weight of the individual and elevates them, like the Gods, from 'common ground'.

On the other end of the scale, Jeremy Deller denies the transformative potential of his proposal. Seeking to present upon the plinth the remains of a vehicle that has been destroyed in an attack on Iraqi civilians, Deller says of his proposal that "it is not an artwork". Deller may verbally be making a politically correct and democratic statement, but for his proposal to materialise in any case, despite art, would create an invasive and veritable arena for discussion about cultural displacement and public curiosity.

Yinka Shonibare's piece engages with the idea of the monument as a way of linking the historic elements of the site with the present social climate. Shonibare's proposal is to make a scale replica of Nelson's ship, HMS Victory, to be housed inside a large glass bottle. The sails made from textiles patterned with rich colours, bought from Brixton market in London, Shonibare hopes to draw attention to the complex journey the material has undergone, in trade with the colonies and in its assimilation in the 1960s as a symbol of African identity and independence. Shonibare's proposal aims to create a dialogue about multiculturalism beginning as a result of Nelson's victory at the Battle of Trafalgar.

Anish Kapoor's proposal draws attention to the plinth itself by making it a support for the five concave mirrors that perch off its faces. In Kapoor's words "they turn the world upside down and in so doing bring the sky down to the ground". Focalising London's changing skyscape, however, is as questionably relevant an act on the square's 'empty plinth' as Tracey Emin's idea for a sculpture of a small group of meerkats. Light-hearted and anecdotal, her proposal is daring, and perhaps a little patronising. Emin claims to offer us a symbol of unity, saying "whenever Britain is in crisis or, as a nation, is experiencing sadness and loss (for example, after Princess Diana's funeral), the next programme on television is Meerkats United".

So who should win? Bob & Roberta Smith have my vote. Their wind and solar powered illuminated peace slogan would look as tacky during the day as it would in the night. But this playfully persistent nod to the thick-blooded military stream propounded through the rest of the Square would both echo a people's revolution and provoke the Square's deafeningly nationalistic chord.