White fish: whiting, pollock or cod are good to start off with. The problem I have is that many older dogs don't go for any fish once they're used to beef, chicken, etc. meals. The key is to routinely feed a fish meal (e.g., once a week) so it's a normal part of their diet. And it's much easier if they start eating fish as a pup.
If they turn up their noses to any fish, you can mix it or smear it with something they really love like raw green tripe and that usually works.