Chatham’s (1707-78) 1766-68 period as first minister (he actually occupied the office only of Lord Privy Seal, but the King and everyone else treated him as leader of the government) was undistinguished. He spent most of the time in Bath nursing his ailments, suffering a nervous breakdown in January 1767 which lasted through the following year and finally caused his resignation in October 1768. He was also almost as difficult to deal with as his brother-in-law George Grenville, and he increasingly left much of the administration to Grafton, outside of foreign policy. For this period alone, he would rank near the bottom of the list.
However, Pitt was also effectively in charge while Devonshire was prime minister and during almost all Newcastle’s second period in office (1756-61). That period was one of the most glorious in British history, during which in the Seven Years War Britain captured Canada from France and gained effective control of India. During this period Pitt was a highly effective war leader, selecting a naval-oriented strategy that was to serve Britain well for centuries, choosing men well and providing inspiration to his colleagues, subordinates and the country at large through his Parliamentary speeches and personality, which was easier to deal with than in his alcohol-and-illness-affected later years.
Pitt was one of three “great war leaders” to act as prime minister, the other two being Churchill and more doubtfully Lloyd George. His son and the other leaders during the Napoleonic Wars do not really qualify; though capable administrators they were not inspirational in the same way (the younger Pitt, because of his heredity and his effective decade of peacetime leadership, initially had some of this charisma, but it wore off under the strain of the late 1790s). Clearly, inspirational war leaders should be given a bonus for this factor. I believe however that the bonus should be limited, perhaps to 20% of the total score. This limitation becomes important when considering Churchill and Lloyd George. Chatham was as inspirational as either, and led Britain in a war that, on a cost-benefit analysis for Britain, was far more successful than either World War, or indeed the Napoleonic Wars.
Like Henry Pelham and unlike most other 18th Century prime ministers, Pitt was personally un-corrupt and left the sordid details of patronage to Devonshire, Newcastle and Grafton. Pelham described him as “The most able and useful man we have among us; truly honourable and strictly honest.”1 On the other hand, his inspiring oratory from time to time shaded into breath-taking populist irresponsibility, notably during the Stamp Act debates.
Given his success in the Seven Years War and the bonus for inspirational leadership, Pitt deserves a high ranking. Equally, he was not fully in charge in his period of greatest glory, and his period of full control was unsuccessful. Still, the edge of the top ten looks about right.
 “Eighteenth-century British premiers – Walpole to the Younger Pitt” – Dick Leonard, Palgrave Macmillan, 2011, p135.