To say the truth. I don't think there is a simple answer to this question. As a creator, personally it sometimes takes me a few months to decide I am ready to share my creation. But when I finally do, it is because I am totally ready to have the world see it.
When it comes to a contest, it is even harder for me to choose that image as I am quite the competitive person, haha.
First I go through what at the time is my best personal favorites. This list contains the images I consider my personal best, with the most perfect lighting and contrasts, facial impressions, body sculpting, interesting styling and the fact that I know it perfectly represents my unique vision and art. I narrow it down from there compared to the latest images shared in the parallel contests that might take place through that same platform as I want to show something new. Only then do I chose out of that small list of images left, the perfect image in my opinion.
This of course doesn't always mean I will win but at least I know the world will see one of my personal favourite and perhaps will like it as well.
And to my fellow photographers out there, please do yourself a favour and try. Take that image, create that creation, don't be afraid of hearing criticism, grow from it. Take a chance and win!