Joe Cocker sang to impress, inspire and touch hearts as well as entertain. Rare are such singers who make people's emotions sway from left to right and make them cry one minute and laugh their hearts out. it is unarguably correct to say that he was blessed with one of the burliest voices in the history of British blues. his body coordination was also superb.
A funny thing happens when a person dies, whether they be a celebrity or an ordinary citizen: the memory of that person suddenly becomes magnified many times over compared to their general reputation while they were still living. In the case of Joe Cocker, before he died, you would have been hard pressed to find his name on very many lists of the greatest rock musicians of all time. Now that he has died, he has suddenly become one of the all-time greats.
Anyone who has ever heard his voice or even his music knows he had talent. He was one of the first musicians to introduce ballads to Rock music. I wouldn't say he was the best singers in history, and certainly not the best voice, but he had a distinct style.
Art, and singing in particular, is so completely subjective that it would be difficult to categorize Joe Cocker or anyone else as the 'best' singer in history. He overcame obstacles and had a very unique voice, and deserves all of the accolades that he has received. Unique and best are two different things though.
Joe Cocker had a distinctive, gritty and yet melodious voice that gave his work an original quality. However, his greatest draw was not in his vocal range but in his stage presence and his charisma. Most people would remember him more as an impressive performer than one of the best singers in history.