Taking into account that fresh veg bought at the supermarket isn't straight out of the ground and isn't usualy all consumed till several days after purchase.
Does frozen veg have more nutritional value?
PS I'm most interested in dark green veg as my doc tells me I need it.
The moment that veg is picked it starts loosing nutrients, some of the veg in supermarkets will have been kept for long periods at low temperatures. The imported veg especially, so generally frozen is best. It is usually frozen so quickly that the nutrients are preserved.
If you are talking about dark green veg ie spinach, just Dont Overcook It.! You are more likely to kill nutrient that way