Apparently this is going nowhere. So I'LL go first.

Gray Fox with his stealth and enhanced spped would be formidable. Undoubtedly, the constant disapearing and reapearing would confuse Tatsumaru. But Gray Fox is not invincible. He can only pop-up and do some damage. It might've been enough to kill Snake(if you lost while fighting him) but I don't think it could kill Tatsumaru. So I will have to go with Tatsumaru.