It depends what your metric is for judging someone to be a good PM or not. From the perspective of being a war-time leader he was excellent; while many of his own suggestions for military strategy turned out to be pretty bad, some of them were good (such as focusing on invading Italy before Normandy), and he was good at picking talented people for his cabinet. He was also second-to-none when it came to raising the morale of the British people in the most desperate times through his combination of gritty determination, charisma and good humour. There are countless contemporary accounts saying that Churchill was the major reason why the British people did not lose hope during the darkest days of the war. It can be argued that this achievement alone surpasses any other smaller matter of state or policy, considering the importance of resisting the German onslaught above all else.

To get a better picture of him as a politician and leader in a more standard, peacetime era, it's necessary to look at his second stint as PM in the early 50s. He was by-and-large a very good PM, both domestically and internationally. On the home front he was eager for postwar unity and thus was heavily inclusive of the Labour party in his government, and consequently the union movements throughout the UK (some detractors say that he gave too much leeway too them, which indirectly led to Britain's future problems with the unions). Though in his younger days he genuinely hated socialism and opposed it at every turn, the experience of fighting Nazi Germany softened this stance somewhat - during the war he eventually said that he came to despise fascism far more than socialism. His second premiership saw the introduction and expansion of several social welfare systems aimed at helping the less fortunate. He also oversaw the gradual recovery of the truly disastrous British economy, although it was impossible to truly fix this in such a short time, and like any country coming out of a war, economic recovery was in some ways inevitable. Strict rationing was kept in place as an economic necessity, which proved unpopular but there was no alternative. He was a fairly hands-off PM on the domestic front, outright admitting he had more interest in foreign policy

Source: Churchill by Andrew Roberts