A word on.............
Frozen veggies.  Frozen vegetables are essentially pre-cooked.  Because I'm not a chemist, I can't use the appropriate termonolody to explain - but think of it this way :  Cooking causes the water molocules inbetween the cell walls of the vegetable to change - so does freezing and then thawing. 

Think about the vegetables and fruits that you **don't** see frozen - lettuce, watermelon, tomatoes (not tomato sauce, whole tomatoes) - they are full of water!  Their delicate cell structure can't take freezing or cooking - they would become mush!  Fortuneately, tomatoes can become tomato sauce when cooked, but never freeze a whole one!

At it's best, you want your cooking method to only mildly change the texture of the veggie, and leave it's nutrients alone.  That's why it's best to cook a veggie in as little water as possible - to keep it's pretty color, tasty flavor, and oh-so-good for you nutrients in place.  Fozen veggie are a great bet - just toss into your sauce or stir fry - and no boiling needed! 

Frozen v. Canned -
I hate canned veggies for a lot of reasons - loss of color, loss of texture, and ruination of the flavor.  A lot of people don't want those added chemicals and salt, either.

Frozen veggies are peeled, washed, maybe steamed, and FROZEN - so there are no added chemicals like salt or monosodiumcancerlutimate.  I vote for frozen every time.

Frozen v. Fresh -
well, this might come as a shock - but Frozen might just be your winner again.  Surely nothing beats a fresh veggie plucked from your own back yard garden, direct from vine to your table - but just how long has that "fresh" veggie in the supermarket been sitting in it's happy plastic bin?  What happened to it before it got there?  Waxes and other preservatives are sprayed on "fresh" veggies in supermarkets; and who knows how long they've been sitting around.  At least frozen veggies are washed & frozen within a very quick time frame.
Often, I'd actually vote for frozen, again.  You sure can't beat the convienince.