The Trader's Point market is all organic, and has stricter rules on how much of the items the farmers sell they must produce. The Carmel market has the most vendors participating but looser rules- some organic, some not. I haven't been impressed with the Fishers, Noblesville or Westfield markets. The Broad Ripple market is popular, and there are multiple markets in Indianapolis, but I can't speak to those because I haven't visited them.
In the summer, the Trader's Point market is Friday night, and Carmel market is Saturday morning- I would visit those two first and decide which you like best!