Metal Gear Solid/Fridge

As for why they used Grey Fox, if you're going to build a cyborg ninja, it makes sense to make one out of the best damn soldier you have available, and from all accounts, Grey Fox was the next best thing at fighting to Big Boss himself. Also, being legally dead meant his rights were forfeit--a living person is going to have to agree to it, and be adequately compensated.
 * Fridge Brilliance:
 * This technically only applies to The Twin Snakes, although this was also mentioned in the Japanese script for Metal Gear Solid, as well. Anyways, Psycho Mantis's line of "It feels so... nostalgic" in regards to performing a good deed for Solid Snake and Meryl Silverburgh before dying seemed very confusing to some fans... until you remember that Psycho Mantis's mother died while giving birth to him, which meant he was referring to what his mother did for him before dying.
 * Not to be bursting bubbles, but the line "It feels so...nostalgic" was actually in the Japanese version of the original game, and it refers to something from Metal Gear 2: Solid Snake. The line was changed to "kinda...nice." for the US localization because that game never was released here.
 * Actually, Hideo Kojima stated in the director's commentary for Metal Gear Solid that Mantis's line was actually in reference to his Mother's last moments being used to give birth to him. Read it here: http://muni_shinobu.webs.com/mgs/commentary2.html
 * I know that it isn't the real reason, but the "Feels good" and the "Nostalgic feeling" set off a different bell for me. See Mantis constantly breaks the fourth wall, right? Well, what's stopping him from doing that while dying? To me, Mantis felt nostalgic in Twin snakes because he already had done a good deed before: In the Playstation version.
 * Also, it seems odd that it required a bribe to persuade Donald Anderson to build Metal Gear REX, especially when he believed in nuclear deterrence and would have allowed for it to be built anyways even without the bribe. However, when you remember that he was Sigint, and that he had a nightmare referring to Metal Gear, it suddenly makes a whole lot more sense as to why he'd have to be bribed.
 * Fridge Brilliance: At one point, Revolver Ocelot mentions being impressed by Snake's skills, and says something to the effect of "Just what I'd expect from someone with the same code name as the boss." The first time you play the game, it's obvious that "the boss" refers to Liquid Snake, who Ocelot was working for at the time. Then comes Metal Gear Solid 3: Snake Eater, where we learn that Big Boss (who Solid Snake is cloned from) once went by the codename "Naked Snake"...and that Ocelot knew him during this time. Maybe it was intentional, maybe not. Your Mileage May Vary.
 * Fridge Logic: Part of Gray Fox's backstory for this installment involves him literally getting resurrected by Dr. Clark and his team. So, yeah: science figures out how to bring someone back from death...and the process is never repeated or discussed again.
 * For that matter, why did Clark even need to resurrect Fox in the first place? Remember, he wasn't trying to synthesize the world's first resurrection serum or anything; all he needed was a test subject for his prototype exoskeleton and gene therapy techniques. He could have just tested those on a living person.
 * Depends on how "dead" he was. If you get to someone fast enough after they receive a mortal wound, yeah, you can "bring them back." Medical history is filled with people who were considered clinically dead for minutes or longer who were "brought back." So no, Dr. Clark didn't develop some magical bring-someone-back-from-the-dead serum, so much as she got to the body fast enough that his tissues were still usable.