The most obvious name is Albert Einstein. He was definitely a very intelligent man but most, it is difficult to say.
It is often said that his IQ was 160 even if true that would make him less intelligent then many people alive today.
If we compare his mathematical ability then his mathematical ability does not come even close to Indian Mathematician Ramanujan. Who was self-taught and invented many functions independently and with other mathematicians? His work was based on the findings of German mathematicians (algebra)
While comparing his works with Famous British physicist Stephen hawking`s work we found that he has better and more depth in topics of Astrophysics.
Then still why we consider Einstein the smartest persons on the earth. Probably he was the first physicist who has successfully delt with mystery of time space and matter in a simple mathematical equation.
Now a day’s psychologist define intelligence in ways. it could be general intelligence and is denoted by G factor.
It could be specific intelligence in certain field and your interpersonal skills.
So, intelligence is a function of many factors in combination.
It won`t be justified to designate one person as a most intelligent person on earth.
Defining human intelligence is a very difficult task, though there are many scientific methods has been evolved to measure the human intelligence