MGS3 is the best game and story in the series, the Boss/Big Boss story is decent and self-contained enough to possibly be covered in a 90 minute movie. With a few re-writes, the characters of Snake and Boss could be expanded on further, giving the actors something to get their teeth into.
It would be a better start to a film franchise than fighting a series of successive bosses and your own evil twin brother, or fighting a fatman on roller skates on an oil rig. No disrespect to MGS1 or 2 there, I like them too, but MGS3 is grounded enough to have the best chance in surviving a transition into a decent movie.