Choosing the right Tuna
There are different kinds of tuna that you can buy in the grocery store. There are basically two options:
Albacore has a superior flavor to chunk light, and it is also more expensive. Albacore is called 'white' albacore because the meat is lighter in color. Not all albacore is pure white, and it may be light tan in color. Albacore is much better tasting than chunk light, but the drawback is higher mercury content. The way around this is to buy a low mercury Albacore that comes from smaller fish.
Chunk Light tuna can be any of a variety of species of tuna. Typically it will include Skipjack, Bigeye, and Yellowfin. This fish is not as good tasting as Albacore, but the price is fantastic! If you are used to Chunk Light, you might want to stick with it, because Albacore is so much better tasting, you might never be able to go back.
The very best tuna is the small, West Coast Albacore that is sold from places like http://CannedAlbacoreTuna.com These fish are smaller and more flavorful than the tuna you buy in the store with the low mercury level of chunk light.
In your tuna casserole, you might not be able to tell as big of a difference in your tuna, so you might choose to go with the cheap stuff, but for other tuna dishes, the small, West Coast Albacore is far above the rest, and you typically cannot find this fish in grocery stores.