What is the best and healthiest way to cook cod?

The problem with steaming is there's no chance for the Maillard reaction to occur and complex flavors to develop. So, if by "best" you mean most flavorful, my suggestion is to sauté with a little oil, hardly an unhealthful choice.

I agree with steaming it. I use bamboo steamers for everything. I made steamed sole on Thursday. I rubbed a little olive oil, salt and pepper on the fillets, wrapped them in foil and steamed them for about 15 minutes. A little squirt of lemon at the end made them yummier..

The healthiest way to cook fish is to simply steam it! I think that if you're going to use the cod as part of fish tacos, that should work just fine!