(And yet it was a close race. We should be ashamed.)
Literally: cloud – mud – ten thousand – ri (a unit of distance, about 2.4 miles or 3.9km)
Alternately: Two things are incomparable. A vast difference. Like comparing apples and anaphylactic shock. A difference like that between the clouds of heaven and the mud of earth, except stretched to over twenty-four thousand miles.
Notes: There’s a cute little phrase, 雲泥の差 (undei no sa), literally a “cloud-mud difference.” It’s so little, though, that I hesitate to introduce it on this site as a kotowaza. There does exist a yojijukugo version that uses 之 (a kanji that long ago served the same function as the particle の does now), but that felt kind of like cheating. Fortunately, the banri version also exists, expressing the same concept in stronger terms… and allowing me to introduce all three phrases for the price of one. 😉
The term 雲泥 is attested from around 900 CE, when it was used in a poem by legendary scholar Sugawara no Michizane. The four-character compound 雲泥万里 followed about a century later, in the poetry anthology 和漢朗詠集 (Wakan roueishuu).