[left of center]

^ The Loop or anywhere near L lines or Metra stations. Of course, this is simply considering the sustainability of such a huge building. Probably would be mixed use, and you wouldn't want half the building dedicated to a garage for residents and office workers. So the less those workers/residents need to drive, the better.

My answer would be the same from a skyline aesthetics perspective as well. Chicago's supertalls are either north of the river (JHC, TTC) or on the periphery of the Loop (Aon and Vista to the north and east, Sears and Franklin Center to the southwest.). Something in the central Loop would really help balance things out, IMHO. The site of the Thompson Center would do nicely be all metrics.