In terms of antioxidant content, the cow's liver appears to rank the highest. Contrast the above number to a beef steak, which is 10. That may encourage you to eat beef liver instead of steak, but the reality is that neither are healthy. While it's true the liver contains more antioxidants, it's not significantly higher than iceberg lettuce. Furthermore, a 3 ounce serving of liver contains an astronomical 302 mg of cholesterol, as it is one of the highest cholesterol meats.
Research Support, Non-U.S. Gov't: Carlsen MH, Halvorsen BL, Holte K, et al. Nutrition Journal NIH Jan 2010