Well if we are talking genius here then it's defintly Uranus (excluding sign/house placements). Sure there are numerous forms of intelligence, even a "jock" has high spatial intelligence. Favorable aspects between Mercury and Uranus imbue the native with the ablity to connect to the beyond self and intuitively understand ideas (neptune as well). Uranus is the connection to the Universal Mind/Divine (in my opinion). Einstien developed his theory of relativity while resting, Tesla got his ideas "out of the blue", if you will. In my opinion, based on much thinking and research, almost any great idea or "eureka" moment comes from the higher self. Now yes this gets a little spiritual but seriously. Uranus also links to the outermost auric layer and the 7th chakra which is the divine. We are constantly being influenced by the divine in ways we don't realize, pushing us in the direction of our paths. Read about Mercury trine Uranus and you will see how incredibly smart one could be. The aspects between Mercury and Uranus have a double Aquarian connontation, which is the scientist of the zodiac (Mercury's exaltation/Uranus rulership). The ideas they receive are intuitive and instant.
For most people to get from A to D we need the B and C... however these people just go from A to D without any long train of logical thought, they just get it (intuitivly). Now, for example, if this trine were to come from the 1st house to the 9th house it would be even more genius. We can imagine any sort of combinations of sign/house to make someone absolutly brilliant. As lillyjgc said, the 3rd and 9th houses are related to knowledge at the personal and beyond self levels as well.
As with anything an aspect or placement alone does not infer anything for certain. Nothing in astrology/psychology is that simple. One of my freinds has his mercury in Gemini, and while hes not stupid hes overly subjective and superficial.
Aquarian energy is far superior, scientifically speaking, then any other. Aquarians get thier own ideas and do not follow traditions as would Sagitarrian energy. Also it is the most objective sign which is absolutly vital for finding the truth. Aquarians are open minded with the potential of accepting new ideas, although yes they can be stubborn in those ideas once they adopt them (it is fixed after all) but no sign is perfect or better overall. And as i said they have a connection and understanding of the Universal Mind which they tune into freely. It is that open mindedness that allows for this.
Aside from that, if one has intelligence and great ideas they are nothing but just thoughts without a means of expression (or desire to do so). The Sun conj Mercury allows for a quick mind and mouth, however this needs to be very close in order for the thought process to be productive and fluid. Also, as was said, mars/pluto entering this equation would provide the stimulus and desire to use this intelligence in some way (Mars for personal and Pluto for collective). I have mercury conj the sun at 4 degrees and while i defintly have above average intelligence there are times when my mind fries out and there are mental "blind spots" and my thought process fails. I also have Mars sextile and Pluto trine, and while this should give me very quick and desisive thoughts i am not the quick minded and witty in normal conversations. Also i am very slow at taking tests, and this sucks for my SAT/IQ scores (i also REALLY want to make sure i am right). My intelligence in the normal sense gets a bit underminded by the Piscean energy. So it is clear that many factors come into describing school/scientific intelligence. Althought strangly enough i am a very fast learner even with my mercury in such a watery sign. Overall it doesn't matter because i just need time to think over things, and i can express them proficently through my writing ability (sorry i know i'm an ego centric Sun in 1st house tends to do that).
Many other planets hooked up to Mercury closely and favorably can provide various intelligences. My new quote describes the necessity for imagination in finding truth and conceptualizing ideas. Imagining a situation can prove as effective as testing it if one has the ablity to think in such a way and posesses the knowledge of the actions/reactions that would ensue. Pluto also gives a distrust that allows for the mind to test and investigate ideas without blind adherence (among other abilities).
Anyway this is kind of long but i hope this helps, overall the concept of intelligence is due to a number of factors depending on the type of intelligence.