Sometimes people get confused with the terms INTELLIGENT and SMART. Just to make it clear, inteligent is someone with deep knowlegde about some subjects and smart is someone who knows shortcuts of solving problems( does not require deep knowlegde)
By experience, mercury in capricorn and mercury in aquarius are the most intelligent. Saturn does great allied with mercury since Saturn is cold and restricted and when it comes to rationality, it important time to think, time to study, wisdom and having little emotinal interference. They have the best for everything. Talking, retaing info, talking in public(Mercury in Cap mainly). They seem and are people who does their best to understand topics. These understand well about the topic they are talking about.
Mercury in Gemini(SMART) is nice for something like convincement, first impressions,best talkers of the 12 mercury signs , but after you know them, you start to notice their "intelligence is more scattered. They convince very well when you know them for not long. They catch little info about many topics so it just helps chats
or storywriting
I can't say much about mercury in virgo(INTELLIGENT), but in theory I think is a very nice one to have(when it comes to rationality and details). They take time to know and put in practice (earth), also mutable as they can change their topics. I don't know many people with this one, but I know one, she is very intelligent.
Mercury in Sagittarius(not sure,but I would say INTELLIGENT) is very nice, but not good for talking nicely,like its opposite,Mercury in Gemini.They can easily say a thing in the wrong time. In academic situations, they are awesome. They easily catch info and the hardest kind ones, the abstract info(the ones hard to put simulation in your imagination)
Mercury in Pisces... not a good one to be classified. They easily say the wrong thing in the wrong time, they don't catch info about pratical easily. Their inteligence is more about art and drama. I know an Aqua with Mercury in Pisces, she is inteligent with the art department.
Mercury in Aries(SMART) is nice for hurry situations, they catch info fast. But like Sagittarius, they can say things in the wrong time, but in this case, they do it knowing it.
Mercury in Scorpio(Pluto and Mars rulership) is good, but not the best. I have this one, it makes me know a lot but about few subjects. To the point I neglect other topics too much. Not good public speaker, since it is secretive. I have a decent IQ, but like all water mercury signs, they are easily misunderstood and clouded.
Public speaking is something difficult to do to someone who has clouded mind by emotions, inteligent people don't get clouded by emotions. How are you going to show/use your intelligence if it gets clouded when you put in practice?
To respectable public speaking it is necessary Sun and Mercury conjunct in the same sign. Doesn't matter the sign. Like I know a guy who is Sun and Mercury in Pisces retrograde conjuncted, he is shy at first but he is one of the best public speakers I know, he even wants to be a politician. All people who does awesome public speaking have Mercury conjunct Sun same sign.