Which European Empire was the most powerful

[The Mikado]

What kind of question is this?

What are you asking to compare?  Military power?  Scope of successful taxation (which directly results in the latter)?  State infrastructure?  Stability?  Ability to have government actually affect the lives of its citizens in a real sense? What time period are we looking at?  Spain's going to fare significantly better in 1600 than 1800, and England significantly worse in 1600 than 1800.