Which phrase suggests that something is nearly impossible or very distant from reality?

The phrase "a far cry from" is used to indicate that something is significantly different from or distant in quality, status, or reality compared to what might be expected. It conveys the idea that the actual situation is nowhere close to what is desired or ideal, emphasizing a large gap between two points.

In contrast, the other phrases have different connotations. "Taste of his own medicine" relates to experiencing the same negative treatment one has given to others. "Will cost you an arm and a leg" refers to something being very expensive, but it does not imply impossibility; rather, it indicates a high price. "Just what the doctor ordered" implies that something is exactly right or beneficial, which is the opposite of suggesting distance or difficulty. Thus, "a far cry from" effectively communicates the sense of something being nearly impossible or greatly removed from reality.