I mean, like with all things how you define ''best'' is complicated...
I voted for Lord Salisbury, partly because of his expert foreign policy but also because people often forget his administrations introduced some important social reforms that had far reaching consequences. Without the Cross Act you can't have any slum clearance in the period or properly tackle urban sanitation problems. He was reasonably capable in keeping the lid on the Home Rule tensions in Ireland, and his second ministry played a big role in reshaping primary education in Britain. And whilst he didn't himself remake modern popular conservatism, he didn't stand in its way and, in fact, did help popularise it.
Its a little bit of a limited list though. What happened to Arthur Balfour for example? Why did Russell, Derby, and Rosebury not make the cut either?