The reason, I suspect, that American food is best is that our chefs don't have an attitude about cuisine. Not the wrong kind, anyway. Our culture, such as it is, is relatively very young, and we know it and so we don't take anything for granted in the culinary arts. We are still learning. I live now in L.A., and I am partial to Vietnamese food. So I Google and find this place called Little Sister. I order a vermicelli with beef, took one bite, and called the waitress over. "What the heck is this?? I never tasted anything so good in my life." The answer was "L.A. Vietnamese fusion," and was it ever! https://www.dinelittlesister.com/ The food actually looks like that.
Either you, or I, or both of us have overstepped the boundary. Let us both be clear about something: American food is NOT the best. Chemical enhancers and sugar (let us not fail to mention sugar!) manipulate American food and the consequence is usually better taste. But often even our perception of taste is being manipulated thanks to those chemicals and the sugar. It is the dependency (yeah, those things the tobacco and sugar industry paid gazillions to registered doctors to claim there are no addictive or cancer-prone elements to their products) that have conditioned us to crave certain foodstuffs. Anyway, I am willing to speculate that with all of that into consideration American food is probably the worst of all. We cannot lie though, American food is the most satisfying to our cravings. We ought to be ashamed that we prefer American food, but we are too cowardly to turn our backs on that taste!
Sure, I mean the best tasting and most interesting food is found in America. There is also plenty of bad American food.
Traditional British food is like any other...it uses what was available before cheap trade introduced more ingredients. Some food today in the UK is , like anywhere else, is due to colonisation like Kedgeree. But today much of the world goes to London, a very important business centre, and the range of food is huge. Lots of Michelin stars. Ditto Paris. There has been huge interest in Far Eastern cooking like Cambodian and Thai, as well as Lebanon and Jordanian food. What is "best" depends on what you like.