I'm a big fan of board games. Or at least I was a big fan; I can't find anyone to play with me anymore. Probably the most fun game in theory is Axis and Allies. However, those games are far too long and far too drawn out that no one I know finishes them (same goes for Monopoly if you play strictly by the Monopoly rules). Thus it doesn't get my vote as my favorite.
Risk is quite good. So are Mystery Mansion and Stratego. There are dozens of other good games that I've played and dozens of bad ones.
But what gets my vote is Sorry. Yes, it is a simple child's game. But get a buch of adults playing it, ones who are more after revenge than winning, and it can be a very, very fun game. Add alcohol and you have another very fun dimension.