assuming they won't go with an original storyline, what game or pre-existing story would you want them to turn into a movie?
Speaking in terms of a trilogy...
1) Outer Heaven (establishes Snake and Foxhound's relationship. you could intersperse the narrative with moments of Snake Eater to get a better understanding of Big Boss)
2) Shadow Moses (brings Snake back into the fold - having to encounter ghosts of Outer Heaven, ending with his commitment to continue hunting down Metal Gears)
3) Guns of the Patriots (the end of Solid Snake)
These are the staple moments in Solid Snakes life. (Sons of Liberty was pretty inconsequential for Snake).
However, if they were to only make ONE MGS movie, I'd go for Outer Heaven, primarily because it is an often aulluded to, but never really seen/explained in the games (not counting the ones made in the 80s). Plus, it leaves it open for possible sequels if we were lucky for that to happen, since its really the begining of Snakes story as we know him.