If I look at the Western Bloc as the countries in NATO, and I ignore for the moment the United States as the strongest military power in the alliance, the strongest country in Europe itself would be West Germany, in my opinion. They were on the front lines (so to speak) facing off against the Warsaw Pact's massive armies, so they responded in kind by both allowing hundreds of thousands of foreign troops to be based there, and kept their military at cutting edge strength. Their economy was also the largest in Europe even before reunification.
The nations of Western Europe were not commonly referred to as the 'western block'. The term 'block' was used to describe the divide between the nations that fell to Communist domination, post World War II and those that did not, commonly referred to as the 'Eastern Block'. Having said that, and to answer your question as I have interpreted it, the most powerful nation in Western Europe during the Cold War was Great Britain. If your answer goes further to the west, it would also include The United States.
The Western powers preferred to refer to themselves as the "free world" rather than the Western bloc.
Unquestionably the strongest country was the United States, which was the only Western superpower. The European NATO allies were all working to recover from World War II, and the US took on the lion’s share of the costs of defense for its European allies. The US was vastly stronger at this time politically, economically and militarily. In addition to direct defense costs, the US also financed the bulk of Western strategic aid to other parts of the world during this time.