I always like to recommend foods as close to their whole, natural forms as possible. Yet, in the case of tuna, it is difficult for me to stick with this principle 100% due to the types of fresh (and frozen) tuna most widely available in the marketplace and their relative risk of mercury toxicity.
Canned light tuna, ordinarily made from skipjack tuna, actually poses a substantially lower risk in terms of mercury exposure than fresh yellowfin or albacore tuna. So this type of tuna—canned light tuna—is the type that's best for you to buy if you want to eat tuna relatively often (for example, about one meal per week). If you are only interested in eating tuna about once per month, other options include yellowfin or albacore tuna (that is either fresh or frozen, troll or pole caught), which I would describe as having a medium mercury exposure risk. While from an ecological standpoint, I would rank these tuna as "best choices" most of us will probably want to consider the mercury and health risks right alongside of the ecological ones when deciding upon our food purchases.
While canned light tuna is my "best choice" recommendation when it comes to canned tuna, it's important to take a close look at the "canned light" label as skipjack, yellowfin, bluefin, and tongol can all be sold as "light" tuna. Skipjack is your best choice among these light tuna options for lowering your risk of mercury exposure.
I also recommend water-packed versus oil-packed tuna whenever you are buying canned tuna. In addition to questioning the quality of non-organic oils used in oil-packed tuna (and the unnecessary, lower-quality fat calories they provide), water-packed tuna, on average, contains a slightly higher omega-3 fat content than oil-packed tuna. However small it may be, it is yet another important benefit that it offers.
There are also more expensive, specialty brands of canned tuna available in the marketplace that may serve as a good option. Some of these specialty products give you better flavor and more omega-3 fatty acids due to higher-quality production methods.
They may also involve more sustainable fishing methods than many other forms of canned tuna.