Fresh vegetables are always the healthiest, and frozen ones are a sad, nutrient-stripped compromise for people who cannot be bothered. Fresh is best, everyone knows that, so the freezer aisle is where vegetables go to lose their goodness.
Frozen vegetables are often just as nutritious as fresh, and sometimes more so. The catch with 'fresh' produce is time: vegetables start losing vitamins, especially vitamin C and some B vitamins, from the moment they are picked, and supermarket 'fresh' veg may have spent days or weeks being harvested, shipped, stored, and sat on a shelf before you eat it. Frozen vegetables are usually picked at peak ripeness and snap-frozen within hours, which locks in the nutrients at their best. Studies comparing fresh and frozen find them broadly similar, with frozen sometimes ahead precisely because it skips the slow decline. Blanching before freezing causes some minor losses, but the freezing itself preserves the rest well. So the freezer aisle is not the compromise it looks like. For anything you are not eating the day you buy it, frozen can genuinely be the fresher choice, which is a sentence that should not make sense but does.