Excluding Russia, since it's mostly existing on an Asian land, rather than a European one, the most powerful nation in Europe would have to be France.
There are three major factors in what makes a state "powerful", and those would be military power, political power and an economy power.
In economy, Germany's economy is the biggest out of all three, with a difference of about 800 billion USD between Germany and France, and nearly a 1 trillion difference between Germany and the UK, making it the 4th biggest economy in the world.
In military, the strongest armed European force is the French army, judging by the factors of size, capability, reliability, quality, training, and above all - experience.
Slightly below France, comes the United Kingdom, with nearly the same skill, training, quality, experience and capability, but a smaller size.
Way below these two European nations comes Germany, with an army that, while still considered "in the same league" with the UK and France, cannot compete them in a simulation of an actual war, mainly due to the quality of the weapons and the power projecting capability of the state.
Politically, the United Kingdom has, quite noticeably, the biggest influence worldwide.
France comes second, with an influence slightly lower than that of the UK.
Germany comes the third, with an influence that is lower than that of France, even though barely noticeable.
Considering all of the above factors, and especially considering the differences between the 'performance' of the nations in each of the above factors, puts the ranking as the following:
1. France
2. United Kingdom
3. Germany
Germany has the biggest economy, but this is the only factor where it is superior to any of the other two nations.
It receives its third place mainly due to its military capability.
The UK is the most worldwide influencing entity in Europe, but its economy is by far weaker than that of the two, especially than that of Germany.
It is overall very powerful, but not powerful enough for the title "the most powerful nation in Europe".
France has a combination of a very well-trained military force, a decent worldwide political influence, and the second biggest economy in Europe, and that is the reason why it is the most powerful state in that region.