I remember when I used to try and imagine how the Sages would fight, use their elements in battle, and other stuff. Rauru would not be much of a fighter, but would somehow have the ability to "bend light" on the spectrum. Saria would speak and telepathically connect with nature. Darunia would be a strong, defensive brawling fighter with fists of flame. Ruto would be agile, graceful, and adept in water environs. Nabooru would have the ability to augment her "fighting spirit", thereby increasing her speed, defense, strength, etc. if the need arises.
Lastly, comes Impa. Besides the way I imagine her fighting ability in battle - which would fit the stealthy, speedy ninja archetype along with the ability to manipulate the shadows, turn invisible, etc. - I also like her personality for some reason. She seems like the "lone wolf type" and she's very tough and stern. One can only imagine what Princess Zelda's training must have been like. Of all the Impa incarnations throughout the Zelda series, Ocarina of Time's is by far my favorite one! So...obviously... I chose Impa.