Halo 3. That said, if you're already got the final battle in your rear view, this is required viewing, as Halo 3's poignant, class and, at times, even moving "Believe" campaign comes to a close.
Though the whole marketing effort has been set after the events of Halo 3, this particular ad is a true postscript to the events of the game. Wow, could we be more general? Sorry, we're really trying hard not to ruin anything. Maybe it would be better if we just showed the clip.