Smoked eel as a delicacy but only very infrequently. (A critically endangered, fascinating species - don't overconsume it like the Japanese!)
Other than that, wild salmon but it's very expensive. Be wary of any of that farmed fish, I've heard and seen first hand some of the ethical and feculent practices involved in fish farming.
Mackerel and herring are best - dirt cheap but because it's so common in the wild you are certain to be getting actual good quality wild fish, not some mutant fed on chemicals and hormones.
Or alternatively, buy a rod and go casting! Give a GetBigger a fish and they will have meal one of six sorted for the day; teach a GetBigger to fish and they will have their entire 16 week pre-contest diet sorted.