Who had the most powerful army in the 1700's?

Overall navy would definitely be the British Empire without a doubt, although the Empire wasn't actually that strong on land.

France alone (not including French Empire against Britain by the same standard would be very close indeed, but an a massive battle between the two including Empires would have to go to Britain due to the sheer size and population of India and other colonies. Then again China had a population of around 300,000,000 in the late 1700's so the lack of technology could be evened by sheer numbers in terms of land units, but they had a poor navy.

Overall I'd say the British Empire.