Bon Chan or Jinbei!
I've read some theories about it being Shirahoshi and some really good points have been brought up, however….she's one of the three ancient weapons, they'd probably rather her stay safe on Fishman Island.
Bon Chan is alive, his use as a SH could be infiltration, spy, sabotage etc although Robin seems to do all that, Bon Chan is even better in this sense.
Jinebei, has been through a lot with Luffy and has already stated he had things to do but when the day comes he might join up with him. I think he will, after he finishes what needs to be done. He's a strong character and made a promise to Ace he'd look after him....It'd be nice to have a fishman around...especially since Luffy falls into the water quite a bit and others try to take advantage of a hammer's weakness, so by having jinbei he will be able to help save DF user's much easier than most of the others. (Just some BS thought's i just put together. )