The extraordinary adventures of Baron Munchausen. This makes my list of most important RPGs because it was the first really freeform game I played. It is one of the few games I know that can readily be played without any reference to the rulebook. Of course if you visit Critical Miss they will worship at the feet of James Wallis the designer of Baron Muchausen and author of the famous Warhammer FRP scenario The Enemy Within.
Mechanically it is one of the most simple games I have come across, with each player having a pool of tokens (usually coins) that are used to reward the player with the best story and in the challenge mechanism. This game must be the purest storytelling game possible with each player assuming an identity as a nobleman and in turn reciting their tale on a topic given to them by the person on their left who was last to buy a round of drinks. While telling their story a player can be challenged by another who wagers a coin which can be won or lost by the active player being able to incorporate that challenge into their story. This continues round the table until all the players have told their stories and the players vote with their remaining coins to produce a winner. Who then buys the next round.
I think this is a great game both for its simplicity of rules and emphasis on storytelling, together with what at least for British gamers is the opportunity to combine RPG with drinking.