They term “best” is obviously subjective and would depend what you’re interested in - art, historical artifacts from specific periods, etc. So not necessarily the “best” but the British Museum currently is the largest in the world with over 8 million pieces of art, history and cultural artifacts. If you take the Smithsonian Institution as a whole which has multiple museums in DC, then they would be the largest at around 155 million pieces. So if you think about which museums would likely have something for everybody then these two would likely be top of the list simply from sheer size and range of the collections.

My personal favorite museum is the Pergamon Museum in Berlin, but I have yet to visit the British Museum myself.