Before seeing Benedict Cumberbatch, I would have answered "Jeremy Brett, clearly". After, I would still answer "Jeremy Brett", because much of the enjoyment of Cumberbatch's Sherlock comes from the show as a whole.
My father seemed to think Basil Rathbone was the greatest Sherlock Holmes, although I'm not sure he's seen much of the Brett version. (For me, the Rathbone version is rather stilted, dated and somewhat balderdash, whereas I agree with the opinion that Brett's version is visually and theatrically a brilliant representation of the Holmes character as appeared in paperform, with the additional benefit that Brett filmed all the Sherlock stories (although in the later ones, he is sadly lacking the energy of the earlier performances).