Since this is a history forum I'll suggest my recommendation for historical artifacts, The British Museum in London. They might have stolen half the exhibits through their colonial period but it is amazing.

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.

The term "best museum" is extremely subjective and highly debatable. It's not a question for which any sort of definitive answer can be provided.