I'm going to be honest with you, I don't think any Mercury sign is smarter than the other. As a matter of fact, the smartest person I know, my father, has Mercury in Libra, and Libra has nothing to do with Mercury.
Each Mercury sign gives you a certain intelligence or ability, in my experience, (And a few astrology books and articles I read) here is how it goes:
Aries Mercury: A quick mind, a bit impulsive, very visionary, they need help in concentration sometimes. I think it inclines more towards Physical intelligence (such as coordination, etc, and no, physical intelligence has nothing to do with the Mars sign).
Taurus Mercury: Gives a Logical, mathematical, and practical intelligence. The Taurus Mercury people I know are very practical and Logical. They might come across as slow when learning things.
Gemini Mercury: Gives a creative (spacial intelligence) yet Logical mind. I don't know how people manage to find a balance between these two intelligences, but somehow they do. They are usually very witty. The ones I know with this placement have an amazing way with words.
Cancer Mercury: HUGE Emotional intelligence, they also have a lot of interpersonal intelligence. They know themselves very well, and know how to read people very quickly, the ones I know with this placement can detect lies like radars.
Leo Mercury: Gives a creative, quick, imaginative mind, I only know one person with this placement, though I read a lot about it... I still don't know enough to truly judge it.
Virgo Mercury: The ones I know with this placement are amazingly intelligent (There is this Leo guy who is younger than me with this placement, and he is fascinated by PHYSICS...) And you know what? He's good at them. They usually have a quick, scientific mind.
Libra Mercury: All Air sign Mercury placements give an ability to see "outside the box", to be creative. My father has this placement, and knows a little about everything. Usually good for book smarts. Libra Mercury is not the best for Mathematical/Logical minds.
Scorpio Mercury: My brother has this placement, and he is also someone whose intelligence I admire. They learn quickly, and usually have a good logical mind. Excellent writers too. They are very deep thinkers, Scorpio is ruled by Pluto, maybe this explains why people with this placement are like this.
Sagittarius Mercury: This aspect also gives a great physical intelligence, along with social intelligence. Usually very creative... People with this aspect usually have a huge spacial intelligence.
This one in my opinion has the most practical mind overall. Very Quick thinkers and with a talent for maths, I'm jealous of this placement, people I know with this placement usually have great grades in classes such as maths, social studies, and history. -.-
More creativity! BUT they might lack Practical or Logical intelligence. They know a lot about few subjects, very intelligent beings, however, the subject has to interest them. If not, they are TERRIBLE at it, and they will not try to be any better sometimes.
Mercury in Pisces is another sign I haven't seen or read a lot about. I'll give it a try. Mercury in Pisces gives a HUGE imagination, no matter what your sun sign is, this will give you a huge creativity. Very good interpersonal skills and Musical intelligence. Mercury in Pisces can make someone rather poetic, and give them a knack for words.
The maximum number of poeple I know who share the same mercury sign is 5!!! So as a maximum I'm basing this off 60 people (as a MAXIMUM, It's probably less..)! So don't think this goes for everyone, ok?
-Virgo with Libra Mercury.