I reckon you have to look at the whole chart and match it to the area of smartness.
There’s mental smartness which relates to the air signs and houses (3rd, 7th and 11th) with Gemini being generally brilliant, Libra the smartest with people and Aquarians the scientific smart that sees things in different ways that are often thought to be brilliant.
Then there’s earthy smart in the way of living comfortably and accumulating goodies. Taurus does this through working hard and being nice, Virgo through keeping everything in order and Capricorn by taking control.
Fire is the smart of vitality and Aries is the first fire sign with its independent exploring enthusiasm that knows no restraint, followed by the proud enthusiastic leadership of Leo and the intuitive fervour of Sagittarius which knows no boundary.
And finally the emotionally smart of the water signs with Cancer looking after familial matters and holding the group together, while Scorpio is fixed on getting to the crux of matters without concern for what gets in the way (then transforming if the object is exposed as negative), and finally Pisces who loves you all and wonders why we have to go through all this again when its so obvious.
To my mind the 9th house (Sagittarius) is the smartest house because of its higher learning tendency and its understanding of spacial arrangement, but the 8th (Scorpio) can give a psychic understanding, while the 12th (Pisces) seems to get intuitive messages.
So what is the smartest intent of a smart person? I believe it’s to live a happy life and that requires vitality (fire), a solid base (earth) a good intellect (air) and a philosophical, benevolent understanding that we are all in it together (water).
The first three make up the structure of the ego and the ego is best expressed by the first sign Aries – ‘I am’. The guiding light of ego is Jung’s ‘unconscious’ which is made up of the personal and the collective unconscious, and is where our moral values come from – as Jung noted ‘the ego can only be damped down by moral defeat’.
In this context I would have to say that the last water sign of Pisces is the smartest when enlightened. Pisces is the last sign of the zodiac so unenlightened Pisces find that they would rather get off this material plane, and their method is drugs and alcohol since the world is such an egocentric cruel place.