One item? I can recommend a handful.
Food--preferably food that boosts attack, defense, and stamina. If your armor isn't up to snuff, some of the enemies (Ganon included) will hit you hard. Compensating with temporary buffs might make the difference between life and death. The stamina boosts will make it easier to bypass the dozens of Guardians patrolling the area and allow you to cut some infiltration time off by scaling walls directly.
Like Azure said, come stocked with shields and Ancient Arrows. Ancient Arrows will make quick work of Guardians, but keeping a supply of shields will allow you to parry their attacks for an easy kill (it's a tricky thing to time, but try parrying a split second after a Guardian's eye flashes blue).
Fairies are also a must. They spawn next to Great Fairy fountains, and if you're in sneak mode, you can grab them without worrying about movement speed. Think of them as extra insurance in case of a fatal blow.