I did Stalin for mine and got full marks in Prelim - it's great to do someone that nobody else is doing in your class, from a time period that may be relevant to your Year 12 course (e.g. Franz Ferdinand or another key person related to WW1) or most importantly a period of history that interests you the most. It also helps to chose somebody well-known as there will be a lot of available information. Really, it's just up to you; somebody with a substantial impact is fantastic and I would say necessary in order to effectively evaluate significance. If you ask your teacher they may give you a list of suitable personalities.