I think each has its advantages: Britain is by far the most powerful country in the world that can reasonably be called a good ally.
Australia has the closest aligned interest.
Israel has demonstrated it will do things against its national interest at the request of the US, and in that it will go the furthest to aid the US. But it comes with a lot of complications for US diplomacy, making the degree that the US can leverage it as an ally the least of the three.
So it really depends on what your definition of "best" is.
Most Powerful: Britain
Most alligned: Australia
Willing to do the most: Israel
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