Here is an interesting news report about the easily available Indian fruit Guava/Koyya Pazham/Amrood.
Dr D Sreeramulu from National Institute of Nutrition, Hyderabad (Endocrinology and Metabolism division) has found in his research that Guava (Koyya Pazham – கொய்யாப்பழம் in Tamil, Amrood in Hindi) is the healthiest fruit while evaluating the amount of natural antioxidant levels of 14 fresh fruits commonly consumed in India.
The fruits researched are: Apple, Banana, Custard Apple, Grapes, Guava, Indian Plum, Mango, Orange, Papaya, Pineapple, Pomegranate, Sapodilla (Chiku), Sweet Lime, Water Melon.
This establishes the fact that costliest fruit is not necessarily the healthiest fruit.
Here is a comparison of the fruits having antioxidant activity in mg per 100 gram. The highest is guava (496 mg) and the lowest is pineapple (22 mg).
For more details, read the Times of India: Humble guava packs healthiest punch.