Fourth Plinth, St Martin-in-the-Fields, London

Times have changed and there are no kings or generals among the six contenders for Trafalgar Square's empty corner – but there is a giant rooster, a musical ATM, and England's royal past rendered as a cake

At the entrance to the show of models for the new Fourth Plinth competition are buttons which, if pressed, allow you to hear the six contenders explaining their submissions.

Punch the top button, though, and you're addressed by a politician rather than an artist. "Hoi," booms a voice, disembodied but bouncy, "this is Boris Johnson"; then, by way of explanation, "Oi'm the Mayor of London." It takes a strong man not to run.

Which is to say that the Fourth Plinth serves many functions, of which the showing of a piece of sculpture is merely one. When Sir Charles Barry designed Trafalgar Square, it was meant to set in literal stone an image of imperial might. The Battle of Trafalgar had decided who it was that ruled the waves, a triumph which left Britannia to build her empire unchecked. In the middle of the national pantheon that Trafalgar Square became was Lord Nelson, his co-admirals memorialised by fountains and their land-based fellows by bronzes on plinths. Generals Havelock and Napier, heroes of Indian wars, glare down Whitehall while George IV, tactfully trimmed by 10 stone or so, sits on a relieved-looking horse in the square's top-right corner. The last pedestal, meant for William IV but empty since 1841, is now the Fourth Plinth.

That times have changed since it was built is summed up by Ken Livingstone's suggestion in 2000 that Trafalgar Square's statues be replaced by ones of people "ordinary Londoners would know". Narrow your eyes and you can see St Jade on Goody's Column, her hand raised in a gesture of blessing. Where civic statuary once elevated people who were better than us above our heads, it now invites us to admire our equals and so, by extension, ourselves. If Big Brother defines the new democracy, then here is the word made flesh – literally so in the case of Antony Gormley's One & Other, which sidestepped the elitism of art by stocking the plinth with real people. Of all the works to occupy the space since 1999, only one, Nelson's Ship in a Bottle, addressed the site's past, which tells you how history is valued in these Channel 4 days.

Which is a long way of saying that the Fourth Plinth is also a place for politicians and arts administrators to show how democratic they are, to temper the vowels of Eton with Essex, at least while people are listening. If Yinka Shonibare was alone in responding to the site's history, he was not alone in spotting its humbug. The best of the works in the past 10 years have mocked the Fourth Plinth project and the egos behind it, the use of the plinth as a venue for talking down to hoi polloi. And so with the new sextet.

Thus Brian Griffiths's Battenberg, a recreation in pink-and-yellow brick of the cake of that name. England's royal past reduced to stale sponge: the idea is as leaden as the gateau and looks like something from Wallace & Gromit. Likewise Hew Locke's Sikandar, which wears its layers of meaning in layers of gewgaws and shows signs of having read too much critical theory. The Cuban-American combo of Jennifer Allora and Guillermo Calzadilla presumably want their pipe-organ-cum-cash-dispenser to elide the sacredness of St Martin-in-the-Fields with the profanity of mass tourism, although it is hard to imagine the result looking good in a light rain. If Untitled (ATM/Organ) booms out notes based on PIN codes, there might also be problems with footpads.

Which leaves the three untricksy contenders, any one of which I would happily see on the plinth. From below, Mariele Neudecker's irritatingly named It's Never Too Late and You Can't Go Back reads as a map of Britain, although sideways-on it's a mountain range. Suitably for the site, the work's grandeur feels like the flip-side of disaster, as though British history is built on catastrophe. Elmgreen & Dragset – what is it with these art duos? – offer a brass boy on a rocking horse, a reminder that the heroes of equestrian statuary (and the dead soldiers in Iraq) were all once children.

Leaving the best for last, Katharina Fritsch's 15-foot rooster in Yves Klein blue has the formal weirdness to make it an obvious winner. While bewildered Koreans watch pigeons pecking grain in Trafalgar Square, Hahn will be poised to peck at them: great fleas have little fleas, et cetera. Of course, a Fritsch victory would mean six months of big-cock jokes on the No 88 bus, but it will be worth it even so.

Next Week:

Charles Darwent trots along to see Eadweard Muybridge's freeze-frame horses and athletes at Tate Britain