How about U+10ffff? It is a non-character, which gives it a high (unassigned character) weight in the UCA. It is the highest code point = "the last character".
It cannot be a Private-Use character, so few people will be tempted to tailor it to something other than its default UCA weight. It also sorts highest in a Unicode-code point order-strcmp. I think that at least in the ICU implementation of UCA, except if you tailor U+10ffff, it will give you the highest weight. markus
