A Metal Gear is a bipedal nuclear tank which features in most of the Metal Gear games. The first made Metal Gear is Metal Gear Raxa which was known as an ICBMG, then there was Metal Gear TX-55 and Metal Gear D. The next Metal Gears were Metal Gear Rex and Metal Gear Ray. After those they finally perfected the Metal Gears and made what is codenamed Metal Gear Gekko.

If you mean story wise then I believe it goes:

Metal Gear Solid 3: Snake Eater

Metal Gear

Metal Gear 2: Solid Snake

Metal Gear Solid

Metal Gear Solid 2

Metal Gear Solid 4

But I may be wrong, and that isn't counting either of the Portable Ops games.

metal gear solid1, metal gear solid 2 sons of liberty, metal gear solid 3 snake eater, and metal gear solid 4 guns of the patriots.

Metal Gear Solid 4: Guns of the Patriots

Metal Gear Saga Vol- 1 - 2006 V is rated/received certificates of:

UK:15

USA:M