Although I have thought about the same question I think that "on Earth" is OK;apparently you don't have to specify by putting "the" because there is no another Earth (capital letter implies too). Though, I suppose you can write "upon the earth" - no capital letter.
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AndreiA person recently visited Dubai. This person stayed there for about 8 weeks. It is a very warm place. She said to me Dubai is the best place on Earth.
Shouldn't it be ' Dubai is the best place on the Earth'?
Compare the slogan of the now sadly defunct Ringling Bros. Circus: The Greatest Show on Earth. The expression is "on earth". "On the earth" is wrong. The BBC article refers to a different use.