Which European Empire was the most powerful

[ingemann]

The Spanish Empire was stronger comparitive to the rest of Europe than Britain were at the height of both empire, in fact unless we only look at UK as colonial power, in which it won, both Spain and France was giants which all of Europe feared.
UK was always just a player among many in Europe and even at it height, it was never a existential threat on it own to its European competetors.
Of course none of these empires was a comparitive strong as the Roman Empire was at it height or even the Frankish Empire.

So I would put it like this with the three choices:

Spain
France
UK