What fruit or vegetable contains the most vitamin C?

It depends on the variety, freshness and soil. Oranges may range from 0-200mg, strawberries redcurrants kiwifruit and persimmons, and many "wild" fruits may rank much higher,see attached link (natural foods, USDA)
In the U.S. The grapefruit. But, the grapefruit is also a fruit to avoid when on certain medications. If you need natural Vitimin C, when on these medications, doctors will usually reccommend pulpless Orange Juice.