That is a tough question
Some people use inexpensive Costco Salmon, but it is not my favorite in taco's ymmv
Southern California's typical fish taco's are
Rock fish/Halibut/Shark/White Sea bass those are my favorites.
Mahi mahi, Ono, Ahi, Albacore are very good but have been priced out of being used as taco filler's (unless its yesterdays fresh lol)
See a lot of Tilapia, Baasra tacos now, because those are inexpensive freshwater "sustainable" fish, and I live in a politically correct area.
They work pretty well even for a fish snob like me
I live on the ocean, so my choices are different than most
My last taco was a smoked Albacore (fresh caught local fish) from a little town north of me called Cayucos, holy smokes one of the best I have ever eaten.