I would say britain

Reasons:
The royal navy,
willingness to use force,
soldiers have proved themselves many times


I think over the next years it will improve further.
When apaches in service.
New carriers built and equipped with f35
Full delivery of typhoons

A note on the challenger 2, well there have been problems due to cost cutting (a tradition in the british forces!!)
But i expect them to be fixed.

Ellie