The Resistance: Hidden Agenda
Buy this firstBGG rates it 7.64 from 884 ratings, the highest of the three expansions. BGG threads call the Assassin roles must-have or strongly recommended and the first module to add.
Needs: Just the base game
Adds: Three modules: Assassin (the Avalon roles rethemed for The Resistance), Defector (spies who switch sides) and Trapper (the leader isolates one team member).