On the Brink
Buy this firstThe one to buy first for most groups. Extra roles alone are worth it, and it lets 5 people play.
Needs: Just the base game
Adds: New roles and events, room for a 5th player, and three optional challenges: a nastier disease, a fifth disease, and a secret Bio-Terrorist player.
Fixes: Base game stops at 4 players, and role variety wears thin. This adds both.