Co-Op with a Traitor mechanic, just because it adds the additional paranoia that someone may be sabotaging the group's efforts. I don't mind competitive, but a difficult co-op game can be the most rewarding.
I've found co-op w/o betrayer to be better. Robinson Crusoe, Pandemic are greater than DoW or Betrayal imo
You don't need a traitor in Robinson Crusoe. That game will murder you without any help. :p