Listverse handily has a list of their top 10 cults. This gives you a little bit of background on each one. The more interesting ones are the ones where murders and suicides have occurred: Jim Jones' Peoples Temple, Heaven's Gate, Aum Shinrikyo, The Manson Family, The Order Of The Solar Temple and The Branch Davidians. All of which have committed atrocities either against others or within their own cult through mass suicide.
I also have a real interest in Scientology because their beliefs are so unusual. They are also quite topical at the moment so it's definitely worth using them as an 'access level' cult for the general public to get to grips with the concept quite early on.
At this point, it might be a good idea to list all the information that might prove interesting for others to read or know about:
-Population (at peak)
-Core beliefs (and religion they broke away from)
-Death toll (own member and other people)
-Why they're considered a cult
And then I'll have to look at different ways to present the information through a bit of contextualisation and messing around.