There are so many lessons to teach a child, it's impossible to just pick one as "most important".
Some of the ones on the list would be:
Follow the rules but don't be afraid to question them if they don't seem right. (prevents guillibility, promotes civility)
Respect and be honest with yourself and others. (prevents improper behavior, promotes social and ethical development)
Know your surroundings so you can prepare for any situation. (prevents victimization, promotes awareness)
Control yourself. (prevents impulsiveness, promotes stability)
Work hard and play hard. (prevents laziness/apathy, promotes motivation)
I could do this for hours. I hope you get the general idea that there is not really one *most* important lesson, it is *all* of the (big and little) ones *you* feel are important that need to be rolled into one.
As long as you are consistent with the lessons and discipline/praise, you can teach a child to be whatever you want him/her to be. Just remember to allow them define themselves. They aren't puppets to be brainwashed, they are independent, individual, adults-in-training. Allowing them to make choices from a young age and teaching them when they made a right or wrong choice benefits them greatly.
Most of these can be taught by discipline for doing wrong and praise for doing right. Using rewards is ill advised because it can promote "covering up" and lying. Making gifts part of thier learning/growing experience works though.
I hope this gives you some insight!
13 1/2 years experience