And the winner of the poll was... Battlestar Galactica!
This is a semi-cooperative game in which you take the role of the various characters from the television series and try to shepherd the human race to safety. However, at least one of the players will secretly be a Cylon, trying to sabotage the fleet. The rules are pretty straightforward but requires the players to work together to figure out how to win the game together as one has many options each turn. It's quite challenging so when you win (together), it really feels like an accomplishment!http://boardgamegeek.com/boardgame/37111/battlestar-galactica