How about capitalizing earth in this context?I disagree. In most contexts, "the best/biggest/most whatever ___ on the earth" would sound highly unnatural to me.
However, you do use 'the' when you are speaking about the earth as a planet (a physical object) rather than as a conceptual unit (a collection of human societies). You'll probably see 'the' used in texts on geology or mountain ranges or something like that.
"Where on the earth is there equal day and night on the Autumnal Equinox?" (This is talking about a physical location on the planet.)