It is obviously Jiraiya with Naruto coming in at second place.
Having the strongest original Rasengan has absolute nothing to do with who created it.
Jiraiya had massive chakra reserves. It's common sense that the more chakra you pump into a technique the stronger it becomes. He was also naturally physically stronger than Minato, Kakashi, and Naruto in base form. Considering the Rasengan is a a technique where you mash it into your opponent, natural strength also plays a part in the total damage done by it.