1. Jimbei. He is currently the strongest fishman and his speciality Fishman Karate is just insanely strong against most of the opponents as it uses water and especially why it's so fearsome is the usage of water in air and in human body itself, so as Jimbei is the best master of Fishman Karate, then he will perform this skill at an insanely high degree. He was able to tie with Ace long ago just with his Fishman Karate and now even after Luffy powered-up so much it still seems that Luffy is a bit weaker than Jimbei, so he seems to be the strongest in this group.
2. Smoker. He was strong in the first half of manga as practically noone knew about Haki, so he easily managed to defeat his opponents as he only relied on his DF to do this. As the only thing in which he bested Luffy before timeskip was his DF being Logia as in speed, raw power and reaction he was considerably lower than Luffy, so I doubt he can handle Jimbei for long as he lost to Law quite easily. Still he wins against Urouge as the guy was one of the weakest Supernovas and he doesn't have a chance against ex-Shichibukai and current Vice Admiral.