if you really want to follow the storyline then I would do metal gear solid 3 snake eater first cause that is when big boss or naked snake was young and on his first mission. Then Peace Walker, that is when Big Boss sort of went on his own forming his own group. If you have the original NES Metal Gear titles then they would be next cause they involve Solid Snake. Then you could continue with the playstation metal gear solid series 1, 2, and 4.
Just FYI, MGS3: Subsistence had MG1 and MG2, which fits in to the canon after PW I believe. The NES game "Snake's Revenge" was a stand-alone American cash in and isn't cannon to the series...but I'd play it just to say you've played 'em all ;)
Metal Gear Solid 3: Snake Eater Metal Gear Solid: Portable Ops Metal Gear Solid: Peace Walker Metal Gear Metal Gear 2: Solid Snake Metal Gear Solid (The Twin Snakes) Metal Gear Solid 2: Sons of Liberty Metal Gear Solid 4: Guns of the Patriots
So if you wanted to play them in order of plot progression rather than release progression, there you go.
play in order of release. that way every game would have some mystery or history than you can find out from a later released game. apart from being brilliant games on their own, having a loose plot point explained in some other release is more fun that already knowing whos who from the strating of the game. just my thoughts... heres the release date order MG MG2 MGS MGS2 MGS3 MGS:PO MGS4 MGS:PW
you could play mgs:pw befor mgs4 as MGS4 doesnt have anything thats explained in PW. (as far as i remeber)