'Superhuman' is the highest difficulty level that can be set normally by a player in X-Com UFO Defence and X-Com Terror from the Deep.
Note that many versions of X-Com UFO Defence have a bug that means the higher difficulty levels are not implemented. You may request Superhuman level but you are really playing on Beginner level.
XcomUtil fixes this bug, and the bug does not exist on some versions of the game - notably "Collector's Edition" aka "UFO Gold".
According to the XcomUtil documentation, there are said to be higher difficulty levels that can be set by hacking game save files, but they are unstable /unpredictable.