Headband

The Headband is a subtle mark of the greatest soldier in the world. The Boss, who was the greatest soldier of her time was first seen wearing this headband. Naked Snake grabbed it from her during her betrayel and ultimately defeated her, signifying her end as as the greatest soldier ever. Naked Snake then wore it for a time before Solid Snake came into its ownership. Solid Snake is known to be the current greatest soldier in the world, but it's expected he will hand it on soon as he is near death by the time of Metal Gear Solid 4.

Something worthy of note is that each owner of the Headband has betrayed their home nation (Solid snake is the exception) and each owner kills the previous owner.