by mtyree1972
On The Brink is an expansion, thus requires the base game.If you have played Pandemic enough times to decide that you like it, then I would say On The Brink is almost a must buy. It adds so much to the game, even if you don't use the Virulent Strain or Mutation challenges. The added roles and events are a giant leap for replayability and really give you some interesting strategy options. Personally, the only part of OtB that I don't use is the Bio-Terrorist 1vAll, but that's mostly because I just don't like that style of play in Pandemic.
The other two expansions, I could see arguments that they aren't necessary (though I have both and would not advise skipping them). In the Lab is my personal favorite but not everyone's cup of tea. State of Emergency is not as vital, but still good.