What is the most important thing in life?

It depends upon the fact that how much of a person's individuality has been diluted.
We are born with a pure instinct which is nothing more and nothing else but love & compassion. All of us carry the same idea and matter in our bodies. It is only the energy which brings the difference between the individuals.
The most important thing in life is to preserve that natural energy, 'that idea' we are born with. Ultimately, what we think always determines that what we will become.
Go on and choose a path, live a life, gain material possessions but don't forget that we can be productive as well as kind and loving, simultaneously.  Don’t act like a black hole which consumes the light; be like a Sun and radiate it with abundance.