I was going to say GTA franchise, but I guess most people already went for it.. So I vote for Half-Life franchise. I can't believe a rather short single-player game could make me feel like I'm being brought to the game for an amazing tour. As far as gameplay is concerned, the plot is solid, entertaining and never bores. Take valve's own Portal for an example.. It is fun with the co-op mode, but the "puzzle" theme gets your hands off the console (or PC) before you know it.