>>218
The MSX one. It will give you a fighting chance to understand what the fuck is happening once the series goes all lolkojima.
Metal Gear Solid. You can read about MG1 and MG2: SS ingame, and few references are made to the games that aren't explained in the game (stuff llike who "Big Boss" is)
Overall, MGS's age does show a little, but once you beat it, you can move onto the more modern titles in the series.