Via “50 foods you should be eating”, papaya is highly recommended:
34. Papaya
If you’re feeling the onset of a cold, reach for this tropical fruit instead of an orange next time. One papaya has more than three times the daily recommended intake of vitamin C and is also stocked with flavonoids, vitamin B, potassium, magnesium and fiber. All in all, it’s good for the cardiovascular system and reduces the risk of colon cancer.
See also the anamed website