In terms of nutrition, fresh is best, frozen is second, and canned is third. You don't lose
much nutrition down the line, but the flavor is usually better fresh. I've reached a point where almost all of the veggies I eat are fresh, and I get excited when I go to the farmers market

If canned is your
only option, then it's still a better choice than eating a lot of overprocessed foods. You might want to read the labels and make sure the canned veggies are sodium free or low in sodium when possible. Fruits should always be canned in juice instead of syrup.