”I first encountered Yoko Ono’s work at a big exhibition at the Riverside Gallery in London. It was some time in the late 1980s, when I was at art college. I liked her work immediately, because it was beyond any genre or categories I had seen before”. Yoko Ono, who turns 71 on February 18, is a household name. But no one knows how good she is as an artist, writes Sam Taylor-Wood.