Well obviously, there are different ways to gauge “most militarily capable”. Are you judging the average “quality” of servicemen (which itself would be hard to quantify)? Are you asking which has the best elite troops? Or are you asking which can come closest to matching the overall power and effectiveness of the US military?

Britain. Easily. The only ally that comes close is France. Both have some force projection capability, but Britain’s is larger and more modernized. As a bonus, Britain is a more reliable ally, fighting alongside the US in more conflicts than France. Britain also has top quality troops.

The only disadvantage they have compared to the US is simply that they can’t afford to spend as much on training programs and facilities, even accounting for the numbers differences.

I feel that they more than make up for this for longer training periods for the average serviceman, however. They also have a much better main battle tank than France, for what that’s worth. Britain will soon have two brand new proper aircraft carriers capable of launching fixed wing aircraft to France’s one.

Although, France’s carrier is a nuclear powered CATOBAR carrier, which means it has more capabilities, the ability to launch higher performance jets (if they choose to buy and use them), greater endurance, and the ability to receive and launch US carrier planes (and vice versa).

Finally, Britain has perhaps the world’s best intelligence capabilities, rivaled only by the US (in my opinion).

But regardless of which is the greatest military ally for the US to have, both are stellar and world class military powers. And plenty of other US allies may not have the ability to wage overseas wars with such autonomy, but have similarly capable equipment and personnel.