Germany has the strongest economy.
The Swiss and the UK have the best financial institutions, although the coming brexit might put a dent into this for the UK
If you consider Turkey and Russia as part of Europe then they have the strongest military.
The Scandinavian countries are the strongest in green energy and sustainability.
I would say Germany, because of their economy, and extreme efficiency.
However, in few years time European Union will become European Empire,
hence comparing European countries between each other in terms of strength,
I find it pointless.
I think the strongest Country in Europe currently is Russia. Under their current president, President Putin, the Country has proven that they are equally powerful to be put at par with other world powers like USA, Britain and China. Why are they my choice when there is Britain: they are loud enough to challenge other world power and their actions suggests the same.