This question is one I have pondered for many years.
I've always had a hard time narrowing it down to just one thing, other than to say that the Lord matters most, since this is His world and we, His creation. But assuming that as a given and looking to what matters most next to Him, to say just one thing is hard.
I think it is of extreme importance to live a life of integrity, to be true to who you are. To do that, you have to know yourself. That can take a lifetime, at least for some of us.
Being kind matters HUGELY to me, that *I* be kind.
Today my answer is, "Being faithful" ... because if I am true to myself, I would also be being kind, and be true to myself, as well as to my God.
Next week I might have a different answer!
Thank you all for your input!