What is the most essential quality you can master? (Your opinion)?
Personally, because it is such a rare quality these days, I think the most important quality to acquire is congruency. Saying what you mean, and then following through with action.
Yes it is a difficult one to master, but if you learn to be congruent, I feel that it will make you a happier person. When someone is congruent, it shows they have integrity, both to themselves, and to others.
In life, the only thing we truly have is our word, that's our bond. And whether or not we keep it shows what type of people we are.
If you can learn the value of honestly and keeping your word, which is a form of honesty in and of itself, it will open doors for you in life that perhaps you never thought were possible.