САМЫЙ ЧТО НИ НА ЕСТЬ - wow! what a great expression. After looking at it several times, it all makes sense to me as literally "the most/biggest ... that there is" but I can't make sense of the НА.

Я знал, что тебе понравится
Насчет "на" мне самому пришлось поискать - что-то из древнерусского
https://rus.stackexchange.com/questions/423402/%D0%9F%D1%80%D0%BE%D0%B8%D1%81%D1%85%D0%BE%D0%B6%D0%B4%D0%B5%D0%BD%D0%B8%D0%B5-%D1%83%D1%81%D1%82%D0%BE%D0%B9%D1%87%D0%B8%D0%B2%D0%BE%D0%B3%D0%BE-%D0%B2%D1%8B%D1%80%D0%B0%D0%B6%D0%B5%D0%BD%D0%B8%D1%8F
В двух словах "на" - это просто усиление, так же как и "ни", т.е. два усиления подряд.

The most - да. В данном случае "самый настоящий"
The biggest - не уверен.
https://dic.academic.ru/dic.nsf/dic_synonims/157640/%D1%81%D0%B0%D0%BC%D1%8B%D0%B9

Wow, thank you. That was really interesting)))

Sometimes we use BIGGEST to mean MOST when we use a noun to describe people: Meanwhile our friend here is the BIGGEST flamer going.