Best Allie - that is a big question. Great Britain, Canada, Australia, Japan , Germany have all been solid supporters of the United States and good places for American tourism, business and entertainment. The old Warsaw Packt countries have become great allies, especially Poland. Can any country brag of a better neighbor than Canada. Canadians have been freat partners in many ways,. Every Canadian I have ever met has been fun to know.
But, if we are talking about a military Allie, that depends on what part of the globe but I would offer the UK all over the world. Reliability, willingness to host American forces, training with Americans and fighting along side Americans it is hard to replace the military commitments of the UK.