Akainu. The only problem I see is Harribel's speed and flight. It would be very difficult for Akainu to hit her. Even buggy managed to avoid him just being a flier and not as fast as Harribel. Actually because of that she might just have enough opportunity to put a tidal wave on him which is kryptonite to Akainu. But I still can't shake the feeling of Akainu winning. Maybe bias? Lol. I want other opinions on this.
I say Akainu as well. And like you said, the only advantage Harribel has is flight and speed, but that really doesn't make a difference. She is a close combat fighter until her release, and when she comes near him, she catches the lava fist
This, and the fact he's also a logia lol.