The Royal Academy is the hottest ticket in town at the moment because of the wonderful and huge show of Johns’ work covering the past six decades. The exhibition is a semiotician’s wet dream with its plethora of numbers, flags, maps, targets and letters. But this is not dry intellectual posturing – it’s also stunningly beautiful. Highlights are the 12 bronze rectangles bearing the numerals 0-9 in relief, each markedly different for the others yet laid out in an identical pattern; the discrete triptych Voice2 with its fabulous splashes of primary watercolour; and the stunning Regrets series from 2014. A couple of years ago Johns was part of a sensational Barbican show that linked him, unforgettably, with the composer John Cage, the choreographer Merce Cunningham, the artist Robert Rauschenberg and pioneering post-modernist Marcel Duchamp. The latter’s influence is even clearer in the RA exhibition because in the neighbouring suite of rooms Duchamp’s intensely thoughtful and challenging works share a display with his friend and erotic brother in arms Dali. It’s a fascinating combination which contrasts the breadth of Duchamp’s vision with Dali’s frankly shallow masturbatory fantasies. The Spaniard may have been a fantastic technician but he lacked his French comrade’s soulfulness and intensity and the groundbreaking spirit that – no wonder! – inspired Johns. If you plan to see both shows, look at Duchamp’s stuff first because it sets the scene both historically and artistically. But don’t spend too much time on the Dalis: hurry next door instead and revel in the genius of the real star of the RA’s season.