It's kinda hard imo, but I think the the 5K would be more OP, it has priority over all high jabs and would win in trades with the nine framers all the time making their 9i useless. 9i 2Ps are OP but would be useless against characters who have attacks with jump(evade low) properties and they could just hop them all day, and faster 2P have less range
9i 6P would definitely be the most OP tho, it has priority over both 5K and 2P...im starting to see why they didn't make nao or Sarah 9i kickers, they'd be so broken...