Lee>>>>>>>>>>>>>>>>Jiraiya's.
Don't get me wrong... Jiraiya's will is for sure up there, it's huge. But he was always a gifted shinobi, we've never heard any stories of his endurance, and Hiruzen was his teacher... his will was only tested during his last fight with Pain. He's spoken about it, but that's the only time he's shown it, verbally.
But Lee had to struggle all throughout his life time, to get to where he's at. His will is probably on the same level of Jiraiya's actually, but I have to give him more credit for it. He can challenge any character in the series, unlike Jiraiya, and even fights when he's knocked out.