I agree that flash freezing means that frozen food are often more healthy than fresh. However, I don't think the same is true about canned. Canned foods often have salt added to preserve them.
it's the next best thing to fresh. it's picked at the height of ripeness and flash frozen thus retaining most of it's nutritional value. dried deteriorates some of the nutrients and canned is usually packed with some sort of salt or preservatives.
Frozen veggies are the next best thing to fresh. They are picked at the peak of ripeness, and processed that same day in most cases. Canned I would stay away from, they often contain preservatives and added sodium.