Our civilisation was very undeveloped before the time of Jesus Christ (hypothetically). Since this was the case communications between group of people (tribes etc.) wouldve been limited.
Even if there was someone who was very popular back in the day, his popularity as compared to the popularity of the famous people in this day and age would be minimal as the worlds population has had a very significant increase in the last 1-2000 years.

The best way to figure this out would be to think of the most famous person as far back as you can go as this persons fame would have snowballed through the last 1-2000 years. Jesus is probably the best answer. However since his life is unproven this may not be the case. The next best answer I could think after Jesus would probably be Julius Caesar.