Rikimaru, for sure. His amazing stealth skills will be no match for Solid Snake's SOCOM pistol, and before he could let a shot off he would be impaled through the chest. His claymore mines would most likely prove to be of little to no use, as Rikimaru's awareness skills would keep him far from them. It would be a quick and simple battle, and as much as I like Solid Snake, he wouldn't stand a chance. Match over.