What is the furthest fall on record?
a far fall
a very far fall
In a sentence you would use 'furthest' over 'furthest most.' "Furthest most' is redundant and many readers would begin to question your intelligence if you were to write that. I would know because I still remember the talking to that my teacher gave me for using it in 2nd grade. If I were you I would stick to 'furthest.'