A number of studies have pointed to the benefits of a diet rich in fruit and vegetables for memory and cognition in older adults. These benefits are thought to accrue from the antioxidants present in these foods, of which some are more important than others.
The anthocyanins appear to be the best — these are responsible for the reds, purples, and blues in some plants. Several studies have affirmed the cognitive benefits of blueberries and Concord grape juice (Concord grapes are particularly purple). Cell studies have also found the compounds in blackcurrants protect neurons from stress, such as that caused by the Alzheimer's peptide amyloid-beta. The darker the fruit, the more anthocyanins, and presumably the more powerful it will be. Another compound in blueberries called pterostilbene apparently, like resveratrol, which is found in grapes and red wine, lowers cholesterol. Yet another compound, called quercetin, has been found to protect against cell damage. This compound is also found in blueberries, and also cranberries. A particularly good source is apples, and red ones are best.
All of this perhaps explains why it is so much better to eat well rather than hope to receive what you need from dietary supplements! And do note that in most cases, most of the 'good' compounds are in the skin. That's why juices (and wines!) are often better ways of consuming these foods.
Of the vegetables, green leafy vegetables, especially spinach, have been found to be especially beneficial. Onions are also a good source of quercetin. Spirulina (not really a vegetable, I know) also appears to be of benefit.
As a rule of thumb, the best fruits and vegetables are those with the most color. And, obviously, it's the color you want to eat (so no peeling your nice red apple!).