Much debate has been about, and always will go on regarding which is Africa's most dangerous animal. From some experience there is no doubt in my mind that it is the hippo. It is a very agile, exceedingly aggressive and very powerful animal. The Big 5 is named as such because they tend to kill you if your bullet did not kill it. The hippo will come to kill you on sight every time you suddenly meet it at under 30 yards or so. In the Mozambique wilderness they are the reason you never drop your guard for one moment when hunting any river bank.
Look at this video of what happened at a South African game reserve over the past weekend when a hippo overpowered and drowned a rhino:
Note that the rhino has had its horn removed by the game reserve's management which some places do in order to prevent the massive, unchecked poaching slaughter of rhinos in South Africa because the horns have huge value in the Far East. Without its weapon the rhino was defenseless, but I have no doubt that the hippo had already decided to drown the rhino.