I've not played Metal Gear Solid 4 but I did see the ending.
Oh man, was that a brilliant ending. The music is astonishing to say the least -- the fight scenes compel you to keep an eye and the seagulls in the backgrou--
Incase you didn't realize, of course there's spoilers.
The seagulls in the background led to this moment of peace and solitude from the rest of humanity -- being as there's only two people in the scene that makes sense. It was almost nostalgic, being someone who vaguely recollects a mission no Splinter Cell, in a location similar to that, with a familiar aesthetic.
Saddest? I prefer not picking a single choice. Amnesia, if you're well immersed in it (as you should be) has a cruel ending (depending which one you pick), Metal Gear Solid 4 is up there, especially when the music kicks in at around 2:48. That is sheer will.
Too bad it's only "saddest", I would prefer ones that have the most impact. . . then I could bring up Starcraft II (and I would link it but nevermind).
Actually, Halo: Reach was quite touching. I can't remember who the guy was but he ended up being impaled by an energy sword and still continued fighting (if only for a few moments after). You end up alone with the aliens and proceed to fight to the death. One thing I think is great for a campaign is that kind of thing -- because too often you need to fulfill the objective whilst staying alive. A good campaign could do well in relying on people being immersed and actually trying to fulfill something, if only to shoot some of them.
- H