Probably Gran Turismo 6. That's Sony's biggest money maker by far and it's still far and a way the sales king for sim racing games. I wouldn't be surprised if a GT6 prologue is a launch game.
No, goddamnit! No more prologues! Just release the damn game when it's done.
Gran Turismo is usually the game Sony uses to show off there new tech, but it is normally just a modified version of the current game. So maybe not GT6, but i could definitely see a souped up version of GT5 being shown off.
But it will probably be more along the lines of simple tech demos showing off specific things like physics, lighting etc.