Fresh fruit and vegetables are best nutritionally when they are in-season and bought locally. During off-season, fruit and vegetables need to be imported. Imported produce is picked before its peak and ripen during transport. They are picked several days or even weeks before purchasing at your local supermarket, than stored in your fridge for another few days. The nutritional value begins to depreciate as soon as they are plucked from the earth, so by the time you actually eat it, most of the nutritional value is lost.
Frozen produce is picked at its peak ripe stage, which is when it is at its highest nutritional value. When freezing, vegetables are usually blanched. This means that they are put into hot water to seal in the colour and ward off bacteria. When doing this, certain vitamins are depreciated. Vitamins B and C are water-soluble and break down during this process.
Frozen fruit is generally not blanched before freezing. The nutritional value of frozen fruit and locally grown fresh fruit are typically the same.
In conclusion, I would recommend buying fresh locally when possible and consume within a few days. If local is unavailable, frozen is a better option than imported.