At the moment, the US - without a shadow of a doubt.
It doesn't matter who else we may feel more of a traditional emotional attachment to, the USA is the only country that has been able to provide consistent, practical, battle-winning support when we have needed it. Those who sneer at the US are invited to consider which of the other potential superpowers would be so friendly towards Britain (and the wider West). It works the other way round as well - some Americans may find Aussie or Irish or Italian culture more superficially attractive than ours but none of her other allies have lost anywhere near as many troops killed and injured as we have. In fact, we are close to having lost as many as all her other allies put together during the war on terror.
Well, that's often the result of shared heritage when that's the case. Americans who are aware of their ancestry share a special kinship with their cousins, be they english, irish, african, vietnamese, kenyan, pakistani, or whatever.
I think militarily though, the US and UK have been the closest as of late. I remember a one of our NCOs talking about his recent experiences in Afghanistan. He said that the allies he could rely on in his personal field experience were (in no particular order) the Brits, the Canadians, and Norwegians (to my surprise and pride as a Norwegian American, he said that the Norwegian SF provided flawless recon and intel, and treated the ANA with more respect than most of the others)
Historically speaking, apart from the Portuguese how about:
India: raised the largest volunteer army in human history to fight for us. Twice.
Canada: raised the largest volunteer army per head of male service-age population to fight for us. Twice. When they didn't have to at that stage in the game.
Our governments have assessed our interests as coinciding with the US's over the last 50-60 years. I wouldn't suggest that automatically makes them our closest allies, just friendly fellow travellers. We have more in common in terms of culture and national concerns with the other North Europe nations than the US.