The most rewarding is 1-AA. Tougher recruiting, playoffs, and great coaches.
The least rewarding (other than the abomination known as DIII), is actually 1-A mid major. Recruiting is generally a horror show when you are in a good world, especially because I play on a casual / semi level, I am trying out I-A now in two different worlds (one should be "easier" than the other simply because of the numbers and NOT because of the high quality coaches).
Namaste.