I'm going to keep this real simple and just include the original 4. For me, it's MGS 1, my favorite game of all time. There was nothing quite like it back then. For me, it had everything: great gameplay, story, characters, setting, boss fights, etc. A plot that kept you interested until the very end. And then you got ending bonuses depending what you did with Meryl, which gave incentive to play again. I really haven't enjoyed the other Metal Gears nearly as much. I just finished Ground Zeroes (lol it really does have an hour of gameplay) and I like it, but the story won't grab me the same as the original.
As for my ranking it's: