It used to be camu-camu, but not anymore. Kakadu plum (Terminalia ferdinandiana) is now the World's Richest Source of NATURAL Vitamin C, 50% higher than Camu-camu (Myrciaria dubia).

Kakadu plum is native to Australia, and contains 3 grams of vitamin C per 100 g of fruit, whereas Camu-camu contains only 2 grams of vitamin C per 100 g of fruit.

http://www.kakaduwildharvest.com/
http://www.rain-tree.com/camu.htm

But gee whiz...it is sooo much more profitable to cut down the rainforests instead. Whoops! Forgot to turn on the sarcasm blocker.

these are unusual fruits because they are not yet commercially grown. To help save this species or indirectly the rainforests, perhaps grow your own, you can order seeds from here:

http://www.b-and-t-world-seeds.com/carth.asp?species=Terminalia%20ferdinandiana%20cs&sref=452266