Pound-for-pound, Lady Blackbird may be the best tabletop roleplaying game ever made. It weighs in at only sixteen pages (including the blank character sheet and two half-page illustrations!) but you could run a ten-session campaign grounded in its single page of setting. Or you can finish the game with a satisfying climax in one sitting. It delivers character relationships, a vivid and interesting setting, a simple game engine with a novel dice pool mechanic that plays swiftly and encourages good roleplaying, and an adventure setup that is open-ended but has a strong narrative drive. You can run it with experienced roleplayers or story game virgins or a mix of both. I have never heard of it going wrong for a player group; I can't even imagine how you could screw it up.
I love it so much that I made special dice for using the system.
And it's free!