At this time of year - December - the most famous, or best-known, or most easily-recognizable, person in the Western world, to Christians, non-Christians, people of any religion or none, people of any age, is almost certainly Santa Claus. Very likely this is the case at any time of year.
Otherwise, it's impossible to define the most famous person, today, worldwide.
By definition, the most famous person must be someone known to the largest percentage of people on Earth today. Santa, in December, must come close, given the above qualifications. Show someone living in northern India or southern Africa or eastern Australia photographs of (US President) Obama, Queen Elizabeth (the second, of England), and Santa Claus, there is little doubt which of the three they'll instantly recognize and first identify.
Leaving out Santa, it isn't possible to answer the question at all because opinions differ, as do the fame of people in different regions of the world..
Michael Jackson
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The most knowledgeable person in the world is Stephen Fry.