I agree that with both frozen and canned you need to check for added sugar, fat, and salt.
Veggies do start losing nutrition once they're picked, but in some cases, canned fruits and veggies can actually have more nutrients than fresh (if the fresh has taken a while to get to market and isn't so fresh anymore). Canned fruits and vegatbles are picked at the peak of ripeness.
I agree though that fresh and frozen usually taste better. Either way, the nutrition is close enough that it doesn't matter all that much. Eat what you prefer and can afford, and consider it good enough.
In order of preference (flavor and nutrition factors being considered)
local fresh picked (from our local farmers market)
fresh from the grocery store
fresh veggies that have gone kind of wilty (I never buy fruits and veggies that look this way, but I do sometimes make soup out of starting-to-droop veggies).
I figure though that even the sort of wilty carrots and celery from the bottom of the crisper that go into soup are still better than not eating veggies.
I almost always have fresh, frozen, and canned on hand, because our budget is fairly tight, so we make some compromises. Also for some things one type of process is better than the other. I love bean salad, and I prefer using french-style beans in a can, because fresher beans sometimes get a weird texture in the salad (sort of weird and squeaky). I also like canned beets, because I hate messing with peeling fresh beets.