Farthest and furthest are both correct superlative adjectives that mean “most distant.” They are closely related to the adjectives farther and further.
Some style guides suggest that “farthest” is a better word for geographic distances (e.g., “During track practice, Gayle ran the farthest of anyone else on the team”).
When you’re uncertain about whether to use farthest or furthest, a QuillBot Grammar Check can help you avoid mistakes.