I'd go between Black Beard's (potentially) or Kizaru's Pika-Pika no Mi.
Black Beard can effectively dismantle a DF user's fighting style since most of them have incorporated it into their combat... making them significantly weaker. Additionally, he can capture people in it, or use it to suck in things such as projectiles. At the same time, he can release things he's absorbed to shield against attacks due to having an atypical Logia body.
Kizaru is honestly IMO the most OP fruit. He has the ability to move at the speed of light... this is something that is LITERALLY IMPOSSIBLE to track or react to. Additionally he can be seen firing off bolts of light, or simply applying Physics with his kicks. Adding Haki onto it causes it to honestly be one of the most unfair fruits if put into the hands of someone who isn't lazy as shit. Virtually teleport and then spam blasts of light.