Play them in the order they were released. If possible, you'll want to get Metal Gear Solid 2: Substance and Metal Gear Solid 3: Subsistence, too; these feature a great deal of additional content over their original releases (Sons of Liberty and Snake Eater, respectively). Subsistence in particular adds an entirely new third-person camera system that improves the overall game dramatically. The second disc also includes ports of Metal Gear and Metal Gear 2: Solid Snake, the original MSX games that began the series before Metal Gear Solid revived it. These aren't necessarily a high priority (and MGS1 recaps their stories well enough), but as games far ahead of their time which set the direct groundwork for the entire series, they're very much worth a look if you're curious.