To answer the question. Depends on what you define as "powerful".
But of the 3, Germany hands down.
Germany's economy is by far the largest and it is still the largest exporter in the world last I looked (at worst in the top 3).
Germany also has the largest standing military force in Western Europe (barely larger than the French but considerably larger than the British).
France has the second biggest economy in Europe, which gives it the second place. This gives also political power along with Germany. Plus its military over all with reserves and paramilitary units is the largest in Europe, which gives also some power.
The UK has next to no real power in Europe due to its Europe sceptic wing and its utter dependence on the US. Basically, the UK is turning more and more into the "black sheep" of the family. Its military might be more active, but it is smaller of the 3. What the UK does have going for it as just about the only thing (but a very critical thing for Europe as well) is the City of London. What the UK also has a considerable impact on is the popular culture of Europe.. music and movies.
But one that is missing on the list is Russia. It is technically part of Europe, and its military might makes it powerful, plus its economic might on especially energy makes it a country to be respected when it comes to over all political/economic/military power.