I don't think there is singly 'one great champion' but there should be a group. And i would put any champion in that group that has done the following..

Defended it a few times agasint true world level fighters, an a fighter who anyone looking back at his record can't find one stand out name that fighter should of fought, 'coz he priced him self out or w/e other reason...

Meet both of those and Imo that puts you in the P4P world champion list greats