In mathematics, there is a technical term extremum (plural extrema) which refers to a minimum or maximum. Your particular example does not involve the minimum and maximum of a graph, which is usually how extremum is used, but it may be appropriate in slightly different situations.
In your particular example I might use the word distant, as in "the two most distant points". Alternatively, if you want to be very clear and precise and don't mind being wordy, you could say "those two points having the greatest distance between them". This is what I would write if it was part of a mathematical proof.