There is no consensus as to that, or even as to what "best" might mean in this context.
Are you asking about universities in the UK, or in some other country, or about the best in the World?
In England, some will say Oxford, others will say Cambridge. Some might say UCL, or LSE. In Scotland, some will say Glasgow and others will say Edinburgh. Globally, some will say Oxford, some Cambridge, some Harvard, some Yale, and so on. That's before we mention universities in civilian law countries such as France, Germany, and the Netherlands, or universities in the Far East such as Singapore or Hong Kong.
In any event, the answer to the question may be academic unless you have the potential to obtain a place at whichever university might be classed as the best. Each of the UK and US universities mentioned above has competitive entrance standards.