USA (United States of America)
Which is most powerful country changes as per time, at one point Mauryan Magadh were most powerful, at one point it was Mughal India and at one time its Britain.
But in current context (2017) atleast from last 50 years and would be next 50 years it is USA (United States of America) which is the most powerful country in terms of Global Fire power.
Top Ten countries in terms of global firepower -
1. United States of America
6. United Kingdom