At White Cube, the walls are yum-yum sugar-pink and turd-brown. Hundreds of pictures in black frames are hung in the shape of two dogs - one defecating, one puking. Overlaid on to the etching series "Los Caprichos", monsters with wide-open crocodile jaws and fangs grin malignantly in Jake and Dinos Chapman's latest assault on Goya, Like a Dog Returns to its Vomit.

Round the corner at Whitechapel, bodies are smeared in strawberry syrup and chocolate sauce, amputated limbs roll around on deck, a platinum fibreglass pink pig displays her teats and bare-bummed, white-teethed American pirates rape and murder on screen in Paul McCarthy's new double installation, "La La Land Parody Paradise" and "Caribbean Pirates".

Apart from Viennese action painting, McCarthy's main influence, has art ever been more orally and anally fixated, or more childishly let-me-shock-you transgressive? The 40-year-old Chapmans and 60-year-old Mc-Carthy alike are arrested adolescents whose wild, clowning fantasies stir us because, for all their surreal strangeness, they tap into grown-up psyches and societies. Both remind me of Günter Grass's tiny Oskar under the table in The Tin Drum, watching adult life from the waist down and sick and terrified by what he sees. At White Cube the result is brilliant, funny, menacing, clever and absorbing: the best contemporary show in London. At Whitechapel it is over-blown, self-indulgent, depressing and pointless.

The surprise of Like a Dog Returns to its Vomit is its unnerving aestheticism. The pink walls look stunning, the bizarre angles at which the etchings are hung add life and interest and the newly coloured reworkings of "Los Caprichos", in delicate twilit hues, are executed with exceptional graphic skill and sensitivity.

When the Chapmans added scribbles and doodles to Goya's "Disasters of War", also on show here, two years ago, they managed against all expectations to enhance and find something new in a Goya who had never stopped being modern and relevant. This time their extension of Goya's grotesque is as illuminating, and truly seamless: where does Goya end and the Chapmans begin?

The theme of "Los Caprichos", that "the sleep of reason produces monsters", was always a Chapman leitmotif. Monsters - apes with Mickey Mouse ears, tendrilled asses with goggle eyes and serpent tongues, silvery, masked clown-animal hybrids with phallic noses - are their thing. There is a lightness of being about these creatures that rests beautifully on the surface of Goya's drawings: now firm, now hazy, captivating in the precision of its detail and refined coloration. The Chapmans take away nothing of Goya's Enlightenment fury at ignorance and cruelty but overlay his satire with the pessimistic ridicule of the nihilistic age to which they belong. The woman on her way to be burned in Capricho 24 is now a guffawing pink monkey, and the bloodthirsty throng surrounding her are carnival revellers and anthropomorphised birds - the peering ostrich retains the feathered hat of the pompous official he replaces. The victim in "Of What Will He Die?", still tended by Goya's doctor-donkey, is a fluff-brain with feathery face and surprised, protruding black teacup-eyes; behind him hoodies with horns materialise out of Goya's shadows.

Shades of Bosch, Brueghel and Dalí are here; so are Viz magazine, Gilbert and George and the century of genetic engineering. As if to mitigate the seriousness of their Goyas, the Chapmans have surrounded them by other clowned colourings-in - this time deliberately inane ones from children's sketchbooks. A dot-to-dot teddy bear spews its innards through a gaping hole in its fur, a clown with vampire nails invites you into "My Giant Colouring Book". Upstairs in "Etchasketchathon" a night owl presides over a dozen decapitated heads and a child plays with a genetically mutated My Little Pony sporting three girls' faces. Funny though slight, these etchings depict a nightmare world - cannibal, paedophiliac - whose terrors, like many of Goya's images, lie far back in the European folk imagination of fairy tale and myth.

It is simplified grotesque, but no less obsessively part of the Chapmans' vision. The dogs may be the artist-brothers and the vomit Goya, to whom they compulsively return, but the title also suggests grotesque's root in an awareness of man's physicality, the baseness and subconscious aggressive instincts to which we all return. Goya, says Dinos, "denotes the point at which art could no longer hide behind religion . . . When you have head and arms chopped - you're just flesh that will rot."

And that is exactly what you are in the crazy carnival of cruelty that is Paul Mc-Carthy's "La La Land". London had a dose of Mc-Carthy's gigantesque banality in 2003 when his 35-metre "Blockhead" and 16-metre "Daddies Bighead", based on a ketchup bottle, became the largest sculptures ever shown at Tate Modern. It is not obvious why we need more, for the really deadening thing about McCarthy is that his sculptures and drawings on the themes of violence, aggressive sex and defecation never show the slightest variety, increases in scale merely compensating for absence of ideas. At the Whitechapel, "La La Land" is inhabited by predictable characters - Dick Eye, a carbon-fibre, shit-brown pirate with a penis instead of one eye, She Man, an aluminium, latex and foam female nude with a man's golden back-to-front head, and a crowd of cartoonish drawings that anticipate the ludicrously overblown off-site island of the Caribbean Pirates at the Coppermill Warehouse.

The only striking work is a cubistic glass ship whose disparate parts lurch at you from different angles, suggesting sea-sickness before you even encounter the buckets of blood and body parts on board the mock-houseboats, and the syrup-splattering films where the pirates play out their colonisation and castration scripts. This is 1960s gory action painting, never interesting in the first place, transposed to 21st-century high-tech installation via pop art and Jeff Koons, and it is dull, dull dull. "La La Land", sadly, is not another country but a highly recognisable one where vacuously grandiose pastiches of Disney, consumerism and American imperialism give the smart set a frisson of politically correct infantile regression, but no one is childlike enough to say that nothing is there.

'Like a Dog Returns to its Vomit', White Cube, London N1. Tel 020 7930 5373
'La La Land Parody Paradise', Whitechapel Gallery, London E1. Tel 020 7522 7888