I thought about this question over and over and I am still finding it very hard to imagine cannibalism. And researching about this, I have found that cannibalism is apparently still very common in some regions in Africa especially in times of conflict eg during the wars in south Sudan and Congo.
My initial answer would be kidney. Because it detoxifies the body, i thought it is more likely to have toxic substances. However, I have just found out that the liver stores high amounts of fat soluble vitamin A which when consumed in high amounts is toxic. There have been a number of cases where people consumed high amounts of dog and other non-conventional animal livers during dire situations and did not survive. So, on second thoughts, Id probably avoid all internal organs