This is a wonderful question to ask, because a writer who reflects on his personal strengths and deficits is ahead of the game. Self reflection provides us a tool in which to gauge the growth of our craftsmanship.
I'm aware that I have several strengths, however, since the question asked for the number one quality, according to our own opinion, I would say it is 'communication'.
As writers, you may assume that all writing is about communication, because...well, writing does just that-it communicates a message, a story, information, emotion, etc. But, in my experience as a reader, there are some folks who know how to communicate what they want to say better than other writers.
Just as there are some speakers who are able to get the message of their speech across to ALL levels of understanding, it is the same with writers. I may not be the most profound writer, but I have a knack to get my message across in a way that says what I want to say, reaches the reader, and doesn't dumb down the message.
Sharing your question.